Dota 2 would probably get Roshan's Pit. After all, LoL's Baron Nashor takes some direct inspiration from Roshan and both super-bosses are very important (technically optional, but you know you want the rewards) to the flow of a match. Besides, both areas are some of the most frequently contested. As such, I'd favor Nashor's nesting grounds for LoL as well.
While super-bosses have been really wonkily implemented in Smash, it's not a horrendous idea in concept. I just believe it needs tweaks in order to have the Super-boss not be incredibly disruptive to the fight (such as it being affected by the Hazards on / off toggle). Both Roshan and Nashor can work since neither are super-disruptive in their respective games (with some caveats of course: I'm not 100 % sure on Nashor, but Roshan ****s people up occassionally if his swipe manages to RNG-stun and occassionally the slam which has a AoE slow).
As for the rewards when each super boss is slain, well, Roshan gives out the Aegis of the Immortal, a one-use item that offers free reincarnation and Full Restore, which is kinda broken and might be an issue, but hey (+ additional items for subsuquent takedowns). Nashor offers a very powerful buff in its stead.
KH might be best served by Dive to the Heart. Reappears often in KH proper, is very important to the main characters, and one can do a lot with the stage. Simple "Final Destination with a Kalos coat of paint"? Check. Wanna have Heartless in the fray with Hazards On? That's possible. More janky platforms for casual fun - again in Hazards On? That works too.