What Could Have Been - On Banpresto's official blog, Word Of God said the team's first choice to round out the Zeta Gundam cast in R was Four Murasame piloting the Psyco Gundam, but because of game balance concerns and a lack of weapons they ultimately went with Amuro and the Dijeh instead.
Macross 7 was also considered for inclusion in R, but the design team couldn't figure out a way to make the "attack with music" system work in A.C.E.. They ended up replacing it with Macross Frontier, which caused the game to be delayed about a year while they changed things around.
In one of his blog posts, Terada mentioned that Super Robots aren't normally supposed to get into A.C.E., but some factors could change that*
Specifically mentioned are popularity and ability to mesh with a 90s/2000s-style serious plotline