MUD suggestions for research purposes?

Are there any MUDs still running, that you would consider to have really game changing mechanics?

I'm doing research while building server framework from scratch, as a pet learning project. I'm familiar with a lot of the basic functionality from playing MUDs a lot as a teen. But admittedly it's been a good decade since I've touched one, and I'd like to find some interesting ones that have pushed boundaries.

Thanks!

EDIT: If anyone is at all interested in following the progress of the project, my github is linked in my profile.