I recently found out that someone made an online version of the Mario vs. Luigi mode from NSMB. They even added new maps and powerups. I played it with a friend for like an hour and it was just as fun as I remember.
Here's the link:
ipodtouch0218.itch.io
Here's the link:

New Super Mario Bros. - Mario Vs Luigi by ipodtouch0218

love this game tbh but its a big ol mess