first for core... and I mean core HOBGOBLIN... it is well past time for a goblin race in the core, and they are most likely to make the cut.
for 'other races' Damphyr would rock, warfordge is cool, Mul, Shifter, and Elan would be nice.
I really would love to see the Dva race slightly reimagined there 'rebirth' mechanic has sooo much potential.
What do you have in mind? 2E style kits? 3E style racial templates? It sounds intriguing. I kind of wish all the subclasses in 5E started at level 1, but it sounds like you have something different in mind.