I’ve toyed with this idea before. It simply to add a car destruction bonus stage in Arcade mode using the car models from FH4. The same vehicular damage that gets rendered when hitting walls could be used for when a KI character beats it up.
There are 3 hurdles. One is that car companies might not like the idea of their product getting destroyed by a fictional character and to that I say it’s all in good fun and they shouldn’t let "car"nage get in the way of them making a a vehicle IRL that attracts the average consumer. The second issue is the argument that it would copy too much from Street Fighter. However, it should be noted that the car in SFV is not as detailed as the 3D models in Forza and based on history, it was only some silver sports car (or SUV in third strike). KI could have a Forza trophy truck like the ones featured in FH4. The final obstacle is that the only linear arcade mode was in Season 1. Either that same mode would need to be applicable to S2 and S3 characters or the bonus stage could be selected from a seperate menu (which doesn’t seem as fun).
Without wanting to raise too much controversy, I would claim that even after Season 3.5, KI is an unfinished game. It could use more polish like adding more animation to TJ’s stiff Powerline or making black afterimage shadows an option and add a few more pieces of content (like finishing Ultimates, character stages and cross-costume accessories).
Since the 25th anniversary of Killer Instinct is coming next year, having this bonus released (and a Season 4 wink wink) would make for a fun experience for both KI and Forza fans.