i guess it depends on what you use tmux for. in another reply in this post i've mentioned detached.el, which allows you to have sessions of commands that live even if your Emacs dies, and it can be seamlessly integrated to your shell, meaning you can use your shell as usual in Emacs but instruct it to run certain commands through detached.el. there are more lightweight alternatives to this, such as sending commands to a compilation buffer (i could link you to those as well, just ping me).

the windows/panes setup can be replicated using tabs and the default window management system for Emacs (which you can tweak to your pleasing). you can even store window configurations (i do something similar to that with tmux).

so what is it that you need from tmux that you would like to see in Emacs?