I think a really good supergroup for Paul would be with:
1. Jeff Lynne, of course (still love Flaming Pie-in fact, still think the Jeff produced tracks are some of Paul's best. Listening to Long Wave and Kisses on the Bottom show that both men have been finding some inspiration in the past before they became fab.
2. Neil Finn of Crowded House. Neil Finn manages to cast his own New Zealand spin on the Beatlesque without sounding derivative like Oasis, which to me is like REO Speedwagon on a speeded up Revolver album; on the other hand, many of Neil's songs are haunting, cosmic and lyrical. However, Neil suffers from staying in the same tempo often and a little lilt and belt from Paul could benefit both.
3. Roland Orzabal (hope I spelled it write) of Tears for Fears. another great songwriter who has delved into his angst with soulful pop, with a just enough fab trippiness. His last album "Everybody Loves A Happy Ending" makes him a suitable candidate for the group.
I hate to sound obvious but a collobaration with Julian Lennon on one track would be a dream. Julian's voice has only gotten better over the years specially with his last overlooked album. Julian sang once on a footstool alongside Tears, "Stand by Me" so it would be wonderful to reprise that track with the supergroup switching out lines.