One good place to look through is the
proposals section of the wiki. There are lots of equipment proposals that would need equipment models.
There's also a never-ending need for more soldier heads to increase the diversity. Destructavator has been working on a new set of soldier models to replace the current ones, but there's no reason we can't have multiple models for unarmored soldiers as well.
More civilian models, more pilots models, new aliens are all needed as well.
There is already one 2x2 UGV model in the data source because they're not yet implemented in the game, but we can always use more. Here's the
proposal for one model.
That's all I can think of at the moment. I hope others will chime in if they know of a particular hole I've missed.