I think they should just prioritize background colours/images as that would clam up a lot of the vocal audience

and that is an easy feature to implement.
These were the top 5 votes from my Rank Your Most Wanted PS4 Firmware Features thread, in order of most- to least-requested (
there are more in the thread):
1. Suspend/Resume from Standby
2. Media playback of files (via USB/HDD, JPEG/MPEG container support; would include Custom Soundtracks here as games already support it)
3. UI customization options (background color/image, display preferences)
4. Custom folders / "pinning" UI tiles
5. Expanded support for Sharing (save to HDD, save video to other services, uncompressed screenshot support)
Notice that 3 and 4 are basically "let me rearrange crap like I want and attach my waifu to background image".
I think filtering will definitely come first. It's a no brainer. And they should implement it as a system wide thing so that most list structures inherit this control. So you can sort or 'float' the criteria you want to the left (maybe ghosting things to the right; there's a few ways to do it).
I do not agree with that mockup UI pic that will probably get posted here at some point, as it is just the XMB rotated 90º, with the PS4 tiles in place. It doesn't work for the 3-layer metaphor as there would be no room to 'drill down' to each game like it is now (which i think is great).
But if they release a big update with the promised Suspend/Resume from Standby, some light UI customization, media playback and DLNA... that would go a long way. Of course they need to shore up a couple of ligitimately missing pieces too, like 3D blu-ray, download priority etc.