I suspect it will be a while before we see the introduction of another full race after MoP given the preparation they require, and unlikely we'll ever see 2 in one expansion now that Panderans are accessible to both horde and alliance. It would make sense to centre future expansions around 1 race and a theme and make that race accessible to both factions.
However both factions sharing a race opens possibilities for minor races to be usable and cross the faction divide. Minor races are tied to major races which a playable, races like Taunka, Mag'har orcs, wildhammer dwarves, ice trolls etc are just variations of main races. Furthermore they don't need brand new starting zones, heavy lore and unique models all the things that really limit the intorduction of more than one or at most 2 brand new player races. You can introduce several as they are far less work than introducing a new race
Minor races can simply be a slight re-skin, start in the major races starting area, and have an npc or two in an independent quest chain told in the starting area or alternatively over several areas to expound on them. With phasing technology availabe, some of the more prominent ones can even have a starting area, e.g. Mag'har can have a level 1 to 6 phased version of Nagrand before porting to Durotar. Furthermore thanks to the Panderan, there is nothing to prevent the minor race of High Elves appearing as an alliance minor race too.
My point? Well, with the new models for all the races being developed, including new animations which is the hardest and most time consuming part of introducing new races, minor races are ripe for introduction as they will simply share the model and animation of the major race but with some simple differences.
Here is an example of how it can look, when doing the re-skin. New models = the new look, including new movement, attacking, casting animations etc. Blizzard have confirmed the new models will not just look different, but also have new animations etc
Orcs: -
Mag'har orcs - new Orc model but with brown skin tones, tribal periperals like headbands/armbands with feathers
Dragonmaw Orcs - black and dark grey skin tones with blood coloured tatoos.
could have class restricitions too, e.g. Mag'har orcs can't be warlocks/DKs or mages, Dragonmaw orcs can't be mages
Trolls:
Ice Trolls (drakkari) - New troll model, dark blue shades
Forest Troll (Amani) - new troll model, moss skin, possibly heavier muscle bulk
New Troll feature - all trolls can grow into berserk trolls when using their racial. -Model same for male/female but keeps the colour of the player troll
Night Elves:
High Elf - New blood elf model without tatoos, non-glowy blue, green, purple, grey eye variations. Starts in phased Quel'thalas before heading to Redridge or starts in Teldrassil.
[or could use a new night elf model instead but blood elf height and skin tones]
Highbourne - New blood elf model with night elf height, skin tones, silver glowy eyes for males and females. Starts in Teldrassil
Blood Elves:
NEw blodoelf modles will finally incorporate tatoos and have more scar options to further distinguish them from high elves
Darkfallen or San'layan: New blood elf model, red eyes, pale death white tones and distinctive facial features, like one half of the face burnt [see SWTOR character creation scar options - San'layan by contrast would have the vampire like teeth and clawed hands , i'm imagining the scar the burn scar options]. Starts as a quel'dorei elf in phased Quel'thalas joining the forsaken at level 6 in Tirisfal glades.
Fel Elf or Wanted: Fel elf will be new blood elf model but with the greenish tinged skin. Starts as a blood elf with a handful of race specific quests, that tell how you turn fel.
Tauren:
Taunka - New tauren model but with Taunka faces. Start zone in phased Borean Tundra/East Dragonblight taunka place before joining Tauren in Mulgore at level 6.
-Edit 25/09- Maulgorian - Tauren model with small varaitions
Yaungol - Tauren from Pandera isle with Yaungol faces but Tauren animations etc
Dwarf:
Wild Hammer Dwarf: New Dwarf model: with tribal markings, if possible a slightly taller and leaner dwarf. Starting zone either phased Twilight Highlands for 1 -6 before heading to Dun Morogh or just start in Dun Morogh. Wild Hammers can't be warlocks
Dark Iron Dwarf : New dwarf model: dark skin tones, red eyes: 1-6 start zone in Thalrassian City aka BRD on Black rock mountain and rest in Dun Morogh or all Dun Morogh. Dark Irons won't be shaman
Draenei:
Broken: New orc model reduced, re-skinned with Broken/Draenei skin tones and Broken tentacles: Animations same as dwarf though. Zangarmarsh or Nagrand start zone for 1.6 then join Draenei at Azuremyst or just start on Azuremyst altogether.
Gnomes:
Leper Gnomes: New gnome model but with Leper skin tones: These are not necessarily green all over, these are half leper gnomes, so sometimes half the face is green or part of the body, and you can have variations. Startzone is same as Gnomes
Cyborg or Mechaflesh Gnome: New gnome model: however the name has been part mechanized: could be an arm or half a face or the legs, you can choose between the variations. Startzone 1-6 phased version of parts of Ulduar then join Gnomes after that.
Gnomes can have a fun new racial called Mechanize ! - i'm told they need an offensive racial anyway.
Forsaken:
New model would be probably the toughest of all the races: No minor race here, as you do get the Darkfallen
Forsaken get instead to choose a death model. A new racial that allows 6secs back to life with death knight ghoul like abilities. Undead you can become are:
Banshee: New model
Skeleton: New model
Ghoul: New model
Gargoyle
Abomination: an option you can become if you are killed close to another forsaken, the more forsaken there are the bigger and more powerful the abomination.
Human:
Like Forsaken, I think a different approach is called for here. There minor races can be:
Half-breeds: Half-elf: New human model but with Elven ears slightly smaller and high elf eyes, could also do a half-nelf which would have slightly longer ears at night elf level, but using the same human model but with night elf skin tones. Starts in Stormwind instead of Northshire Abbey or could start in Terokkar Forest Allerian Stronghold Phased 1-6
Vrykul: Hylanaar Vrykul: New human model but Vrykul height - not sure it's worth redoing the vrykul model just for this. Starter zone could be Stormpeaks phased 1-6 near Thor's peak, before joining the humans in stormwind.
Black Humans: Human models but with dark skin tones and affro hair styles. I know hairstyles aren't easy to do, but if they're doing new models they might as well. This could be a mystery when a large group of humans turn up in stormwind claiming old blood ties because there home ist destroyed, - or we could make Kul'tiras a pre-dominantly black nations that has a pseudo like feudal/democratic system, every dysnasty has 3 generations in charge before it is changed... well put it this way it can be something new and imaginative.
-Edit 25/09- cultist human - human model but with weird markings and scars
Goblin:
Spider eyed and Coal goblins: Goblins aren't getting a re-model, true, doesn't mean they can't create new skins though wtih different eyes for spider eyed goablins and skin for coal goblins.
Gilgoblins: starter zone Vashj'ir phased for 1 -6 before joining Goblins on Lost Isles.
No Worgen or Panderan minor races I can think off - they won't be getting new models and it doesn't make sense to create a minor race for them like I did for the Gnomes.
Disclaimer: I've just given the ares that would make the most interesting starting zones for the minor races, I would be fine with all of them simply starting with their major races. Class restrictions are really easy to implement. Racials could be unchanged or borrowed from other major classes and modified slightly.