Good morning MMO-C!
Woke up this morning to what I feel would be an awesome idea for how to implement starting new characters of any race at level 20: Story-driven classes.
Think of SD classes as "allied classes." You need a character at level cap to earn them; you need to complete a quest that explains why you have them; and you need to experience their beginnings by progressing through some portion of the expansion. Once unlocked, you can start playing them at level 20, and each comes with its own "heritage" armor.
Worgen Paladin -
Crowley never intended to help Eyir in Stormheim. He just wanted to piss off sylvanas. Regardless, the Valkyrie sends some of her followers to help retake Gillneas once and for all. They show up and fight back Sylvanas', leveling the playing field. At the end of the quests, one or more of them who were wounded decide to give up their lives to grant the worgen a closer connection to the light. By being "purified" in the eyes of Eyir, you can now start a worgen paladin at level 20. Your heritage armor looks like a valkyrie/vrykul and your running wild ability is upgraded so you look blessed by the light when using it. Alternatively, you get a Wolf mount (alliance wolf, yes!) from the plains where Fenrir was as a nod of appreciation from Odyn.
Example - Orc and Human Druids
Remember on Draenor, there's an orc druid who talks to you? Suddenly, she shows up at your garrison with a quest for orc and human characters to travel into Gorgrond and stop the Overgrowth from spreading into Tanaan and through the portal that links it to Stormwind. Throughout the questline, you are gifted druidic powers and control druidic abilities through vehicle interaction. At its climax, the orc sacrifices herself at the heart of the overgrowth, wherein you find a nascent Draenor wild god who offers to teach your kind how to harness the power of the druids. You now have orc/human druids as an option and may start them at level 20. Your heritage armor is a leafy set that makes you look like one of the plant humanoids you've been fighting. Your mount is one of the bug creatures like the store mount but colored differently.
There would be one quest for each race, adding something that they didn't have before and allowing you to start them at level 20 going forward. Once you earn your "heritage" armor, it becomes available to all members of that class on your account, so you can still have your main, you just have to level up to cap with another character to get the special armor.
More Examples
Gnomish Paladins -
Gnomeregan is free! But there are still leper gnomes in the bowels of the city who need your help. They haven't completely lost their minds yet, and Mekkatorque wants you to try and save them. As you travel down with a priest NPC, you learn more about her and watch as she must turn from her pacifistic ways to help the lepergnomes before they are lost. Before the climactic battle, she picks up a dropped sword and shield, as well as retrofitting some armor from a fallen mecha. When she's ready, she says the light in her has changed. You finish the quest, saving the lepergnomes, and when it's done she invites you and any who were saved to join her in learning more about her new gifts.
Congratulations, you have unlocked gnomish paladins. They start at level 20 and their heritage armor looks like it was cobbled together from a mech suit. Their mount is, of course, one of the mech birds, but jazzed up with cool chrome finish and golden exhaust pipes, etc.
Dwarven Druids -
Something in the Hinterlands is causing the Hatching of the Hippogryphs prematurly. Joining some wildhammers, you are sent by Avianna to learn why. As you quest, you find out that harpies are trying to warp the creatures to their own, twisted reflections. By stopping them, you earn the respect of Avianna, the wildhammers and the hippogryphs. As thanks, Avianna travels to the wildhammers' home and helps them learn druidism. Now you can start a dwarf at level 20 as a druid. Your heritage armor is very reminiscent of the wildhammers and hyppogriphs, and your mount/travel form is one that was purified during the quest.
Draenei Druid -
The Krokuun of Argus have made their way back to Azeroth. Separated from their homeworld and unable to wield the light, they turn to you to help them find a new path. unlike the broken of Outland, the krokuun do not hear the elements whispering to them. Instead, they find solace in helping Cenarius heal the Emerald Dream, which has areas that remind them of Argus before its corruption. As you help cleanse the realm and introduce some creatures to it so they survive this new world too, Cenarius grows to appreciate your help and that of the Krokuun. In exchange for your success, he takes the krokuun under his wing and offers to help your people learn the druidic ways as well. You can now access draenei druids at level 20. Your heritage armor is a blend of argus and emerald dream. Your travel form/mount is an emerald coarser like those you can get randomly in Argus.
Pandaren Death Knights -
Taran'zhu sends you and one of his warriors to Kun'Lai to recover shards of the Divine Bell that was destroyed by Garrosh. They thought it had been recovered in its entirety, but with the bell reconstructed, they now know something was lost. Traveling to the site, you discover the death knights have also traveled here in search of the shard. They want it to help raise more of their kind and become fully independent of Bolvar, who keeps butting in on their business. At first, you're against the death knights, racing them to where you believe the shard is, but ultimately, you have to join them in defeating a mogu who discovered it first and is trying to use it to reawaken the Sha of Anger under his control. During the fight, the pandaren with you is slain. Afterward, the death knights give you to shard, saying that its whispers of rage are too close to those they hear already and they would rather avoid more voices in their heads. So, you use the shard to speak with the pandaren's spirit. He agrees his time is not yet up and rejoins his body as a death knight. With taran'zhu arriving (late), he dislikes that this has happened, but agrees that a balance must be struck. If anybody could bring a balance to rage and death, it's his people. He agrees to allow the pandaren to raise more, so long as they are willing. You now can make level 58 pandaren death knights who start as spirits and pick their side (alliance/horde) upon resurrection. At level 60, you are set free from Kun'Lai and asked to help your people however they may need. Your heritage armor looks like some of the Samwise art for pandaren DKs and your mount is an undead turtle creature.
Tauren Warlock -
In Highmountain, you join Magatha Grimtotem at what is to be the last battle with the Blood Totem. Arriving at the location where this is believed to happen, you find out that some of the tribe are trying to fight back a pocket of Legion demons who were holding out for orders that are never coming. After questing with one particular blood totem, you learn that not all of the tribe turned willingly to the Legion's ways, and Magatha points out this is similar to what happened with her people. She says you can make the right decision, where once the wrong conclusion was drawn. These blood totem are unable to go back to their normal ways (some prefer to fight fire with fire), and instead offer themselves to train your people to try and harness something good from all the bad that went on. You get access to tauren warlocks at level 20. Their heritage armor is grim totem and blood totem inspired, and their mount is a demonic moose or kodo, depending.
Troll Paladin -
The zandalari aren't paladins as a people. Their beliefs are too foreign. However, trolls are introduced to new loa in Zandalar who have closer ties to the light because of the Titans mingling there and dealing with Gh'uun. After some questing, you meet up with a handful of prelates and free thinkers who left the zandalari ways to pursue the light and learn it from their own Loa. This brings you on a trip to several locations of troll power including the Throne of Thunder and Zul'Gurub. After finding a home for the loa away from the blood trolls and the zandalari zealots, it thanks you by offering to train your people. You can now be a troll paladin starting at level 20. Your heritage armor looks like a mix of zandalari plate and traditional paladin gear. Your mount is a blessed raptor.
Blood Elf Shaman -
You are called to meet with Rommath, who was recently training fire mages, only to have them mysteriously disappear. Tasked with finding them, you make your way to Hyjal, where you discover that they have been lured to the Firelands with promise of easy power. You manage to free one before the rest run off, and she explains that the elements themselves have spoken to her. She travels with you deeper into Ragnaros' homeland, where the two of you fight off an insurgent group looking to dispute the new fire lord's rule. As this continues, you encounter other elementals sent by the other lords to aid you under their new agreement to work together. They comment that you are already too set in your ways, but this fledgling is ripe for learning. Ultimately, you defeat the group, and the former mage leaves on a tour of the other planes. When you return with news and the saved elves, Rommath thanks you and speculates that the girl may have done more than just save her people. Indeed, she returns in what will be your new heritage armor and offers to start training the elves who no longer want to only focus on the elements of fire, cold, and arcane. You can now make blood elf shamans at level 20. Their mount is a firelands hawkstrider.
Goblin Monks -
It should come as no surprise that goblins want to corner every market they can, so a group of them have set up a "dojo" in the Krasarang Wilds, near the old horde establishment while they were there. Here, they are trying to learn from the pandaren, but are too impatient. Naturally, this leads to strife, which as is prone to happen here awaken sha to the area. By purging their greed and angst, the goblins eventually reach enlightenment in their own way. You can now start goblin monks at level 20. Your heritage armor is similar to chi-ji and your mount is a crane.
Example - Undead Paladins
As the expansion goes on, you (an undead) learn of Kel'thuzad's resurrection and attempt to recreate his own 4 horsemen. He's already recovered Zeliek, who you free and help defeat him. To prevent this from happening again, Zeliek guides you through a questline to finally find and destroy Kel'thuzad's phylactery. After you have done so, he apologizes that the taint of your work so far has cut you off from the light in a way that suits his training, but offers to train others in your name. Congratulations, you have earned Undead Paladins and can start playing one as a level 20 character. Your "heritage" armor looks like Zeliek, and you get a mount just like his.