I think that sounds a really interesting change. We shall each have time to think up some books, and decide whether we want a theme or a random selection.
And I think the honour of doing the first one should go to sun surfer as he's our Fearless Leader and he thought of this!
I have had a look again at the current list of themes and while it's just about impossible to decide which to drop and which to keep, here are my suggestions - not worrying about which month each would be in:
Drop:
Highly Challenging
Lottery
Open
Contemporary
Region (because there are two)
Poetry
Keep:
Plays
Region
Time Period
Non-Fiction
Obscure
Short
But I would be just as happy with any other configuration. And of course if someone's pet theme was dropped out, he or she could always reinstate when it came to her/his turn.