what games hav good core ideas but implement them bad lee

my pick is messiah
able to take control of other characters is neat in uuuh concept then, but is borin', poor lee designed, buggy an' in general broken

wasted potential

I think the poster child for this is The Elder Scrolls Chapter 2: Daggerfall.
Great game, actually but even better potential. A massive living world where mass battles take place as political under currents threaten to swallow the player character whole. And we get a buggy mess with broken dungeons and thousands of samey towns and faceless, nameless unmemorable NPCs with broken dialog trees.

Omikron https://www.gog.com/game/omikron_the_nomad_soul has a similar mechanic with possessing different characters. Been forever since I played either so I can't really recall how comparable they are in that regard.