Well, I think the most important thing that makes 3rd Gen hacking popular is AdvanceMap, amirite? So once we have a map editor for D/P/Pt/HG/SS that can do all the functions that AdvanceMap can (the important ones), then 4th Gen hacking will become much more popular.
I think these are the most important features of AdvanceMap that makes 3rd Gen hacking popular:
*Edit map layouts
*Rename locations
*A nice GUI that's very easy to understand, such as when changing OW sprites
*Built-in wild Pokemon editor
*Very easy to create/assign scripts
Correct me if I'm wrong, but PPRE can do everything on that list except for the first two, correct? PPRE can change overworld sprites, but it's annoying since you can't visually see which. PPRE can change the wild Pokemon of any map in any 4th Gen game, and PPRE can also edit scripts and move their X & Y locations, correct? I guess it's "hard" since you don't have a visual reference as to where the scripts activate, etc.
So I guess what's slowing 4th Gen hacking is a map editor with a nice GUI. I think it would be easier if people were willing to "keep track" of all the events. I mean, think about it, in 4th Gen hacking, we already can do most of the things that we do in 3rd Gen. PPRE can basically become the AdvanceMap for 4th Gen games, we just don't have a map editor/viewer to combine it with so that we could have some kind of a reference when placing scripts, etc.
I have no knowledge of 3D or anything, but since there's a map editor for Super Mario 64 DS, then that means a map editor for Pokemon Diamond/Pearl/Platinum/HeartGold/SoulSilver shouldn't be hard, considering Super Mario 64 DS doesn't have grid-based maps.