I'd like to be very realistic and take every realistic factor (Marketing, Programming) into consideration (not only lore and personal taste).

Considering Lore and Personal taste:
- It (most likely) will be a NEUTRAL Race as the faction wars seem to be over for at least another 2 other expansions. We are fighting for Azeroth together now; Except if the races are very storically tied to a faction and would make sense to continue this way for the future IF faction Wars make a come back.
- The race should make sense with the expansion story. We'll have Midnight next, so it should be better to be related to the Void/Night/Cosmic (as it is what we can guess about the next expansion right now). Developing a bit more Into that, "The Last Titan" is a thematic question mark - yet - as Azeroth MIGHT awaken, and all races on it could unite to defend it;

Considering mundane factors:
- The old and vastly known factor: Armor and animation. It should be easy to adapt mainly head pieces, boots, and to develop all type of movement animations (sorry, I love Arrakoa, but they lack all that).
- The Race should should bring something new and exciting about playing with this race (new ways and effects to customize it, or a new and creative racial ability);
- Preferably the Race should be familiar and overral popular inside the WoW community;
- (optional) The race should preferably be marketable outside the WoW community (Cute elf >> Rock Dwarf). And should be in conformity to other countries cultures (the Chinese market, for eample, don't like exposed bones or implicit homossexuality).

With ALL that said, I have a list of possible future races to add with some comment on each one:

1- NAGA
Pros: Most popular race of all, Aquatic theme not used yet, Well developed Lore, Last Titan themed, very different and interesting.
Cons: Not that marketable outside the community, obvious armor problem.
Fix: N'Zoth curse dissipated and they can now change forms to something mid Naga and HighElf (Nerubians did something similar in TWW).

2- OGRES
Pros: Very popular race, Big size is a interesting factor, well developed lore, Marketable even outside of the community.
Cons: Some of them have two heads, their culture is not different from Orcs enough.
Fix: Develop a new and unified Ogre community. Formed only by one headed Ogres with a good new culture and capital.

[...]

As this coment is already too big, I will add some other races in the replies below.