Final Fantasy VII added many new summons to the roster, but it would be cool if there were monster trainer materia, akin to the !Catch and !Release commands of FFV, or (less popularly) Gau's Rage command in FFVI.

That's the thing about FFVII though, it stripped out a lot of the clutter the series was building up at that time, and delivered a more action-packed experience.

That being said, I would like to have seen Ultros as a companion to Typhon (however its spelled). Missed opportunity to add a regular character to the series, imho.