I would love to see an option that's similar to what we have now with the per-project audio folders, except also including a Picture Cache sub folder as well (and like the current setup, have the option of using a global Picture Cache like we do with the global Audio folder if we choose to not use per-project audio).
I've been doing a LOT of work lately and my Picture Cache is quite full, especially from a few very large projects with long tracks in them. I've bumped up the PicCache size to help with that but eventually it needs to swap stuff out and it makes redraws of new recordings slow down a fair bit. The new SONAR definitely helps with how smooth this is though - in the past it would cripple my aging system while it was redrawing. Having per-project PicCache would help with that dramatically, plus you could just empty the current project's cache without having every project redraw if you emptied the global cache.
The other advantage of this is when you're opening up an archived file. If you also archive its PicCache then BAM - you're basically back to how you left it with no waiting for waves to draw, and when you're done, archive it all to your backup drive and delete it from your work drive, and no left over WOV or WTR files cluttering up your drive and slowing down drawing of other projects.