A solo artists version makes sense and I see people have been suggesting it for a while in this thread. Obviously that means changing the format to 'comeback' rather than reunion. So far, we have no indication that that's the case so can only assume it's still 'reunion' as in, people coming back together.
Most likely there are some 80's bands/groups they could pull in. I think most of the 90's/early 00's ones have all been done. Those that actually warrant a reunion.
Bringing solo acts together to form a supergroup just did not work last time and should be avoided at all costs again. Saying that, we all know we would cream our jeans for a Billie > Samantha Mumba > Rachel Stevens > Holly Valance album!
The only thing I can see is them reaching out to more recent disbanded acts, like JLS or The Wanted. True, they haven't really been away very long but they were at least quite successful and I personally wouldn't mind seeing new music from them.
I'd say people like Phixx would get a look in but they didn't every really have a hit first time around, did they?
I'd love to see Hepburn but, again, I'm not sure they were big enough, despite a massive lead single and a decent album. Maybe it doesn't matter how big they were as long as they have a story to tell.
Republica? M People?
People saying All Saints should do it - no... All Saints have worked so hard to get to where they are right now. They are releasing great music and touring and gigging. It would be a bit of a backstep for them. Plus, they've already reunited, like years ago so I can't see the point. I'm pretty sure they were contacted beforehand (years ago) about doing the show anyway and they declined.