Shouldn't be too hard. Build it on GTK or whatever. The hardest bit is translating escape codes into screen updates.
Some things I'd like to see, of varying plausibility:
- Display keeps up. Pine is unusable on gnome-terminal, it's pathetic. I want responsiveness!
- Anti-aliasing. It's nice.
- Adjustable padding. Make character cells a few pixels higher or wider if the font isn't sensible about it. Do something cute with non-monospaced fonts, let cells bulge or something.
- Smart color choice. I want to be able to choose a foreground and a background, and have it adjust the other colors so that they have sufficient contrast against the two. If the foreground and background in a cell clash, damnit, adjust one of them and to hell with consistency of palette.
- English syntax highlighting. Give the vowels a different value. Give "of", "the", "a", etc a different color.
- Fisheye around the line the cursor/mouse is on.