Creators of Super Smash Flash Making an Official Indie Crossover

Fraymakers looks to be the most ambitious indie crossover fighting game yet, with a successful Kickstarter campaign that is still ongoing.
Successfully funded on Kickstarter!

Indie Games Spotlight - Flipping the Switch

Masahiro Sakurai Says Work on Smash Bros. Ultimate Will Be Done After Releasing the Next 6 DLC Characters - Siliconera

Jake's Super Smash Bros. - Fantendo

The Indie Platform Fighter Taking On Smash Bros - Fraymakers

Indie Tournament! (An Indie Smash Bros Game) Fan Casting for Make your Own Crossover

The 4 best games like Super Smash Bros. on PC - Softonic

Fraymakers (an indie platform fighter by the developers of Super Smash Flash 1 and 2) adds a Switch port as a stretch goal : r/NintendoSwitch

So, Who's Next For Super Smash Bros. Ultimate? - Feature

Indie showcase: Super Mario Bros. Crossover < NAG