Will there ever be Items/Attacks/Spells that can summon enemies in battle as allies?

There is not currently any support for summoning enemies in battle. It is possible that a future version might support this feature, but no specific plans have been made yet

Fortunately, you CAN make this (sorta) happen! You will need to sacrifice an enemy slot on EVERY formation. Make an enemy called or terrain, make it untargetable by enemies, make it have 1 hp, no speed at all, no attacks, and change it's bitsets to "Ignored for alone AI" and "win battle even if alive", and place it on EVERY freakin formation, right in front of your allies, in slot 7. Then, change EVERY attack and spell that any ally or enemy will EVER use to "Cannot target enemy slot 7", Except the spells that summon the monster. Make these spells Transmogrificate that enemy in slot 7 to whatever monster you want it to become. You will probably need to make a special "Summon Specific" version of anything you summon, with attacks that target it's allies, or heal it's foes, as the game will not actually recognize the summoned monster as part of your party. Make sure that when the summon disappears or dies that it spawns the "terrain" enemy again, or you'll only be able to summon once per battle. Yes, it is a HUGE pain in the rear, but it works perfectly, if you're willing to put in the time to actually do it. The only snag is that there is no way for the actual enemies to target the summoned ally. And hey, no messy plotscripting! Sorry newbs, if you don't have a clue what I'm talking about. This ones for the Pro's...