With all the stuff they fixed/added with DmC DE, they need a HELLUVA lot of stuff to match or surpass that.
But thing is, in a Special Edition you can only fix gameplay stuff. I would love to see actual Dante levels, new bosses etc. but it's a Special Edition, not a full remake.
BUT, unlike DmC, DMC4's combat doesn't really need the same number of fixes/adds the DmC DE is getting. The only thing I could think of is more moves and weapons for Nero.
Instead, the heavy weight for DMC4 SE will be Vergil. No matter if he's just a playable character or if he will get a campaign. Cause even in the worst scenario, him just being a playable character, that would already be a surpass in itself. It won't just be a bunch of gameplay tweaks, it will be an entire new character to the game to play as. The only concern would be his arsenal's extent.
And of course, if on top of that, he will also get a story (which is doable, unlike remaking the main game's story), well...
The more I write of it, the more hyped I get so I better stop here... LOL