Honestly, I would see Raid scaling (either raids up, or us down) as one of those ideas where people are like OH YEA SO COOL for maybe a month... three months? Then they fall by the wayside again. I'm not saying it won't be this (it's more likely than most things I've seen in the thread), but I think many people actually enjoy going back and soloing as much of old content as they can. As it is, my guild is pretty much dead outside of various raid times (there's 4 raid groups in our guild that all run at different times/days), and no one ever seems interested in going back to do achievements they missed, let alone a full raid.
I would be 100% on board with cross-faction grouping. I have many friends on horde side on my server where my main is ally, and would really enjoy even just doing the odd 5 man with them when I can.