Wouldn't it be cool if we could have hi-res fonts, status bar, inventory keys/items, etc...? Ya know, all the small bits of graphics . I think this would be cool .
There has already been a set made (for JDoom and related ports IIRC). I don't like everything in it, but some of it is OK and it is all there (I think). For Doom at least.
Also, as far as making them goes, it's probably less of an onerous task than making a new bunch of sprites for a single monster, or something.
Since the 2D texture drawer can recognize scaling, yes. But of course you have to be careful with a software renderer. The higher the graphic's resolution the more visible the scaling artifacts will become.