This ROM hack for Mario Party 3 on the Nintendo 64 adds Bowser as a playable character. It would alter the game's dynamics, introducing Bowser's unique abilities and playstyle into the minigames and party-style gameplay.  The changes implemented by the modder and the specific gameplay tweaks would need to be sought from ROM hacking communities or relevant sources.

Playable Bowser in Mario Party 3

Play as Twilight Princess Link in Ocarina of time