Price of admission: run in a terminal, run graphically, run on every possible system, be open source, be ridiculously, easily extensible.
What a successor might add:
Not be riddled with bugs.
Not be ridiculously, terribly bad at actually displaying text (quickly, smoothly, large amounts).
Not be extended in a language that's so slow that most "intelligent" modes end up being piles of half-correct regex hacks.
Not not care about correctness, in general.
Hop to it, dear idealistic genius whiz-kid reader!
Or fund me, overly-$-laden startup lottery winner. I'll take a crack at it.