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Maybe The Dev Team Thinks of Everything
Or a variant of Video Game Achievements?
Edited by DAN004 MAX POWER KILL JEEEEEEEEWWWWWDefinitely not The Dev Team Thinks of Everything, unless the game is intended to be Unwinnable by Design. I hardly think "winning" can be considered unexpected on the part of any developer.
Cosmetic Award (to which Video Game Achievements is a Pot Hole) fits, though. It's a reward that serves literally no function other than to indicate that you've achieved something.
I feel like I've seen this specific variant at least a few times, though. It could be a Sub-Trope of Cosmetic Award.
Edited by SolipSchism

Is there a trope for videogames that change the appearance of the title screen after certain events in the game, most commonly after you have beaten the game? For example, Devil Survivor puts on the title screen the sprites of characters whose endings you have seen, and Nier, after you've seen the last ending, adds there a flower.