Well those droideka from star wars would be nice. The rolling ones with the two laser arms. But they could be the fast moving close range unit with the scyther filling the long range. They could also drop a small personal shield or something to research to make them. Maybe just add a separate shield hp bar. The b1 battle droid could be an early attack followed by the heavier b2. Some kind of exploding bomb bot could be added to break turret lines or maybe walls. Some quick melee bot would be nice when melee weapons or something is added or they could grab and shock your colonist to stun them for a time until they are shot off them. Could also do those stealth bots from Xcom that strangle your guys. There are a lot of ideas you could think of from other games.
There comes the problem. Sorry but this is NOT star wars.
Having a certain type of mechanoid with a personal shield might be a good idea, yeah, but I highly doubt simply copying others' creations is anywhere near an acceptable idea, let alone an outstanding one.
I for one think about a balanced mechanoid between the centi and the scyther (HP, movement speed, range, firepower, all between the two of those...), and a really squishy melee unit that could attacks in swarms would be nice.
Also the already suggested repair drone and (maybe mobile) artillery would be...well, not exactly fun for us to deal with, but spectacular anyway. ^^
But they should spawn regardless of the map type imo.
One thing, whatever kinds of new hostile mechanoids may take place in the core game, modders will work wonders for us to use them as allies, no need to worry about that. ^^