The best possible thing: Knytt Stories editor on the PSP

Srsly. How hard would that be? I might even be interested in making something like that, if I get ahold of a PSP SDK or something. Would I need MMF to do that?

An editor without a pointer device? I think it'd be too cumbersome to use. Unless you meant to say the PS Vita. There's homebrew stuff for both portables now, but there's no MMF port that I know of; I know you can export MMF2 software to Java, though, which should work on PSP. There's also my own editor in Java, but I don't have a PSP to try it on (and frankly I'm not really interested).

An editor without a pointer device? I think it'd be too cumbersome to use. Unless you meant to say the PS Vita. There's homebrew stuff for both portables now, but there's no MMF port that I know of; I know you can export MMF2 software to Java, though, which should work on PSP. There's also my own editor in Java, but I don't have a PSP to try it on (and frankly I'm not really interested).

Mmm, not with the PSP. Circle to swap between tilesets, Triangle to swap between choosing tiles on the tileset and placing the selected tiles, X to choose tiles to place and to place tiles, etc. I'd think it'd be pretty simple. Even then, you can use the analog stick to move a pointer, like how the PSP Browser is. (Though I'd prefer the DPad to choose tiles. :v)