1 already kind of exists. just go to analyze on a custom variant and hit space for the computer moves. It's certainly not the best moves all the time but they're never terrible. there's also a website called fairy stockfish that's what you've described.
My Ideas for a variants update
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1: fairy stockfish: allows you to play any variant in any moment and also allows you to play your own variants and custom positions
2: create custom pieces: allows you to make custom pieces using the movements of pieces that already exists and/or drawing arrows on the board to set the movements, set the value of the piece and using a png image for the image of the piece