That's right.There's Planetes coming out at the end of April
It's currently due out on Monday 26 April, with the Gurren Lagann simulwatch set to run until Thursday 29.
Maybe Planetes should be before Ergo Proxy, then? It's been on the suggestions list for longer as well.
Planetes is 26 episodes, so that would account for most of May. How about June for Ergo Proxy, @Lordhippos and @WMD?
I think that @Geriatric hedgehog might have had the idea of that being an October one, for Halloween.people have also suggested Garden of Sinners
I've got another suggestion for a short October simulwatch that could either precede or follow it. I'll wait until a bit closer to the time before going into any specifics, but it's a non-anime one, a trilogy of Korean films. (I wonder if anyone can guess it. )