If you're going to do something new, you better stop worrying about backward compatibility. ssh and tmux won't work? big effin deal. Those projects would need to grow up in order to keep up with the times. You can't be conservative like that if the goal is to right the wrongs and learn from past mistakes. The road is going to be bumpy but the light at the tunnel would be worth the travel.
I say if the limited scancode thing is in the keyboard hardware then we need to fix the hardware itself as well. New keyboard standard that sends Ctrl/Shift/Alt as separate scancodes. Hell a completely programmable keyboard firmware sounds even better. Throw in some n-key rollover and now we're talking!
I would like to see some convergence of terminal and windowing-system happening. I initially dreamt of a graphical terminal but then I thought why not go one step further and make it the main interface (so something in the middle of a graphical terminal and a tiling window manager). However I still need to carve out the details.