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!

Fraymakers - an indie crossover platform fighter from the people behind the Super Smash Flash games

Super Smash Bros. Crossover Ultimate, Video Game Fanon Wiki

Indie Pass : r/SmashBrosUltimate

10 Best Games Like Super Smash Brothers

Rumour: Fresh Sources Suggest Shovel Knight Is Indeed Digging His Way To Super Smash Bros.

Indie showcase: Super Mario Bros. Crossover < NAG

Super Smash Bros. Crusade Now Has Over 60 Characters, 24 Stages, And An Online Mode - Siliconera

The 8 biggest rumors of E3 2017: Elder Scrolls in space, open-world Star Wars, and more - The Verge

crossover with indie characters (what do you think?) : r/ShovelKnight

Indie Smash: The Ultimate Indie Crossover Game - Build the Roster