I feel like Blizzard has learned a lot from this expansion (as with previous expansions) but I feel like they made a few mistakes, at least, for what this game is like currently for me.
I enjoyed scenarios, once. But they seem to have be pushed into a higher level of content than they are intended. Since MoP beta I have always felt Scenarios are more intended for levelling content, rather than end-game, but I don't think they should be exclusive to levelling either. Dungeons feel more like they should at end game content now, but instead of dungeons, they are pushing more scenarios.
What I would like to see in the next expansion is a good combination of both sides of the coin.
Scenarios with their quick queues and smaller number of people and their great storytelling properties should be implemented for levels 90-95*. If 2 scenarios are added per level, thats a total of 10 scenarios and group content (aka group quests) with a decent reward of EXP and a levelling piece of gear. Repeatable daily for high reward, or in the same day for a lessened reward.
Scenarios could even be used like the IoT ones and only be available for certain quests, and not be returned to afterwards. These could be unlimited in quantity and give less (or no reward) before the quest handin reward
Along the same line as scenarios, a normal mode dungeon or two for each level (totalling 5+* dungeons) between 90 and 95* could also be added. Requires a longer queue (or organising a group with guildies) but rewards for role specific loot. Pretty much the same as today's levelling dungeons.
This is all for levelling content, so per day (for full rewards) you can do 2 scenarios, 1 dungeon and quests for EXP to level from 90 to 95.
At max level, this leveling content will unlock and become heroic mode. So now at max level you can access this content through queues or portals, providing 10 scenarios and 5+ dungeons already accessible at max level. Throw in some exclusive max level dungeons or scenarios and you have a lot of content to clear through and provide gear pre-raid.
Give scenarios an exclusion list (like battlegrounds) and a specific loot table for heroic scenarios and you now have a reason to go back and do certain scenarios before acquiring raid gear.
*(This assumes 95 as max level for the next expansion)
TL;DR: Normal scenarios and normal dungeons as levelling content, heroic scenarios and heroic dungeons as max level content
(The current formula is dungeons as levelling content, scenarios as 90 content, heroic dungeons as 90 content, heroic scenarios as 90+ (higher gear level) content)