While the cynical side of me might think it doesn’t matter, Moore has burned an awful lot of bridges so I’m not sure the big publishers would welcome him Prodical Son wise. They’d sell some books, but the cost might be high.
More than that, to write for the big two, or any branded comic really, you need to somewhat color within the lines. Not only does Moore not color there, I don’t think he sees the lines anymore. So if he were to do something, the condition would be it’s set in its own space and he can do what he likes. He wants to play with the toys the way he wants to. And again, I’m unsure if big corporations would welcome that. They might get a hit, but that hit might hurt what they have right now.
I think the big name writers of this era - Moore, Gaiman, Morrison, Ellis, Millar, Ennis, BKV as examples - would find it very hard to work elsewhere, which is why so few of them are getting big pay checks at the big two and instead getting small paychecks at the publishers who’ll let them do what they want. I think for all parties getting back together and trying to make magic is too difficult. That in part is why when they do come back they just tred familiar ground like Miller of The Dark Knight Returns sequels, or Ennis on Punisher.