well at the moment units can be designed with equipment from research or other sources.
It's not so different from galciv really, you can equip a trade module, but not a trade and construction module... or a hat, but not 4 different hats... also armour, weapons, sensor modules (scouts cloak)
Still, it does leave a lot of scope for other potential modules/equipment/bodyparts to be added. I suppose if it's not too difficult to do you could probably add extra limbs... but it doesn't really work visually unless you increase the size of the torso... and there's armour compatibility problems... which means having different defaut chassis for certain modifications, since none of the equipment fitting a light frame will fit a giants frame.
There aren't seperate unit design screens for monsters/summons... so probably from a set stock... but we haven't seen any of the editing tools for buildings or anything else yet, so we may be able to mod in custom looks if nothing else for certain summons. It'd be nice to be able to have that in the design screen though, so you could do it all while in the game... but it's probably early days for most pretty 3D things yet... they're for later... now it's the cloth map and simply thinking about flying in a z axis.
I want them to have flight in... and it's quite likely... but I really really want a 'land' option... it really bugs me that if you're able to fly it's assumed you always want to do so... but in x-com again, flight makes you a big target for ranged attacks (no cover) and also makes it hard to aim ranged attacks. Those are the sorts of things that should matter. Fighting farmers? fly with a bow and arrow, fighting archers? fly away! or *run* and hide (from the first volley at least). Maybe you don't want everyone to run away because they can't hit you. Land and stand your ground with armour able to reflect damage... you need them to be able to hit you first.
Depending on the damage, order of attacks and retailiations, flying could be disadvantageous by disabling retaliation damage, changing your damage potential and armour or ignoring those factors completely. It's annoying sometimes because it's with no real reason for doing so... other than it's assumed you're always going to fly a few m above them. (I'm just thinking that I have no idea what the tactical battles will be like, but I assume they're flat... so you'd need flying to be a toggle button with invulnerable to short weapons while active and penalties to both "to-hit" and cover related defence.)