I'd like to see Insectoids. Not 'Nids, but actual bug-folk.
No, the Vespids don't count.
For a change, I'd like bug-men WITHOUT a hive hierarchy. Give them chitinous carapaces for a good armor save (the game could do with more 4+ base save units- we don't see very many around here), squad leaders with what are effectively lightning claws, maybe the army doesn't include a single model that moves as base infantry- they're all jump infantry, beasts, or monstrous creatures. Insanely good initiative, high number of attacks, good weapon skill- but no power fist analogues and "only" Strength 4 for the infantry-base models.
To balance this insane speed, they'd have the worst shooting capabilities the game can provide- possibly none at all at the troop level, just for the hell of it.
Of course, this reads a lot like Genestealers with Extended Carapace
Honestly, any race I could think of adding is either already in the fluff and has been passed over, or wouldn't really fit.
After all, the 40K-verse is already pretty damn crowded.