I'm also not sure what's "wrong" with existing terminal emulators, except render acceleration when you do a lot of text I/O, but I think urxvt already had that solved.

Other items seem to be solved outside of the terminal like 'bookmarks' or 'connection managers' and 'profiles'. Some emulators seem to focus more on style or discoverability, but one of the upsides of the existing learning curve is that you have to mentally organise and plan what you were going to do, instead of trying to manufacture a solution in the moment.