Mechanical Improvement - Split Final Smashes
Final Smashes have been divided into Super Smashes and Final Smashes. Super Smashes retain the standard function, being powerful attacks that are obtained by breaking a Smash Ball or filling up the meter. However, they are a bit toned-down, having less range and damage than the old Final Smashes. They're closer to the Supers of more traditional fighting games, to allow for them being used more often. Final Smashes retain the name, but are closer to Ultimates, being extra-powerful, extra-flashy attacks, with most (Or all) of them being cinematics. These require three Smash Balls/filled meters to use, being represented by a three-tiered counter on the fighter's HUD. To allow for this, Final Smashes now have a separate button. Notably, the flashing lights and monochrome background of the old Final Smashes only occur with the new ones.
Graphical Improvement - Unique Shading
The aesthetic of Infinite is shifted towards a slightly more exaggerated, cartoony style, at least proportions-wise. This is emphasized by the cel-shading, which varies between fighters. Goofier characters are more cartoony, darker characters look like they're framed in shadow, etc. This also applies to the stages, really emphasizing the crossover aspect.
EDIT: The cel-shading would generally feel like somewhere between a comic book and a detailed sprite, like Spider-Verse and Guilty Gear, respectively. As Venus suggested below, you could also toggle between the styles of past games, or even have different styles made specifically for Infinitie (Cartoony, comic book, shadowy, etc.)
Granted, these would be some pretty big shakeups. If you aren't fine with these submissions and don't put them up for consideration, I understand.