It wouldn't break my heart to see an FPGA agnus replacement that could address more than 2mb chip to do more in DPaint (the only painting/drawing app I ever, ever "mastered"), so that I'd never get those dreaded "Derp you're out of Chip RAM!" messages (or when playing large MOD files, or for moving bigger textures around in FPS's on the Amiga - yes I like them there too)...
I wonder if it would be fun to pin some electronics on my degree work, give it a stab...hmm...
Ahem, anyway. A soft 68k core running at some insane clock-speed would be a must. PowerPC based Amigas do zip for me.