Ar Tonelico X Ragnarok
Or Gust in general, that'd work. An Atelier x Wizard outfits.
Maybe a crossover with their future projects?
We can dreamI don't see why not though, you see collaboration among Japanese developers quite a bit. Trinity Universe, Cross Edge, Keiji Inafune's head in Hyperdimension Neptunia Mk2.
That's because Idea Factory is the king of crossovers (maybe because the company itself lacks originality, in my opinion).
It could be, but there are other examples. Street Fighter X Tekken (and Tekken X Street Fighter), Nobunaga's Ambition X Pokemon. I know Idea Factory uses them quite a bit, but all I said was that it was not uncommon to see crossovers in Japanese games. Hell, Right Racer is getting idolm@ster DLC.
Crossover with the company's own franchises are more likely than in-between companies, that's why Idolm@ster has been featured from Tales, Ace Combat and Ridge Racer.