FFVIII has a whole slew of summons that would be interesting to see. Off the top of my head, there's Quezacotl, The Brothers, Pandemona, Doomtrain, Eden, and technically, Griever.

Doomtrain and Eden would probably be the most interesting as there isn't that many inherently mechanical summons and both of those summons feel like something Garlemald could come up with...