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I understand that season is stretching it a bit, but I think this could be the general idea?
I live to breathe
This is difficult without having more context on these shows and what constitutes a grand finale.
I know Futurama was canceled and Un-Canceled once or twice. So are you just talking about a Series Fauxnale or two?
Series Fauxnale is what OP is asking about. For works with true multiple endings see Multiple Endings
series keeps going after the finale = Post-Script Season
...because the series keeps going on or there's a very important event. ''Phineas and Ferb'x has one for the 4th season, but the show also has few otger epiaodes that count as this, like "Summer belongs to you" etc
Same with Western Animation/Futurama, where the show has about four of them!
Edited by Jet200