Well, unpopular choice, but I would like the player (Terrarian, as I call it, despite the yo-yo existence.) to join Smash, I know it won't happen, but because that 1.3.6/1.4 update is coming, along with a Switch port. and had Japan exclusive content, had also a 3DS port, which makes him have some chances.
His moves could be based out of weapons and bosses. He wouldn't be just a swordfighter, because there are many possiblities for his moveset. His Final Smash could summon Moon Lord to fight for him, just like Piranha Plant's final smash. Skins could be NPCs.
Terraria has some nice music, the stage can be any biome , or biomes that keep changing. If spirits (not fighter spirits) DO happen for DLC, it can be enemies, NPCs and bosses.
If bosses for DLC characters happen, Moon Lord or Wall of Flesh could be it.
And imagine seeing Terraria in 3D.