I wouldn't expect anything in the base game to tie into any existing IP-- that's not how PG generally operates (with IP tie-ins left for expansions and DLC).
Some sort of legally-distinct-not-a-Gundam seems like a likely candidate for a Japan-themed showcase, though. I'd put my money on something kaiju-adjacent showing up, too (and, if they're going to do both, it'd probably be worth putting in the engine work to make them both stompy-- although their general aversion to showing car damage means that you're probably racing against the giant stompy robot/monster running beside the track rather than being on the track with it, similar to the train showcases in previous games).
