Needs Whedon on board.
Also, there is no way to express just how conflicted I am over this. While on the one hand Firefly is perhaps one of my favourite shows ever of all time, I love Nathan Fillion in castle, and would hate to see him leave that show.
I don't know much about people doing several shows at once but from what I've heard it's very rare outside of cameos, and often leads to all sorts of conflicts. Firefly only lasted for 14 episodes, not even a full American season, which meant that it still had time to become absolutely awful. Sure it could also have gone on to literally be the greatest show of all time, but would you really take the risk now? We got 14 episodes of the highest quality and most fun of almost any tv ever, and I wouldn't want to risk that memory getting soured seven years down the line.
Castle is a good show right now, and so far it hasn't shown any signs of slowing down or breaking that run. Whedon is notorious for eventually having his shows turn to death terror and heartache, and the amount of people who complain about seasonal rot after even two seasons of Dollhouse should make all fans very very wary of anything Whedon goes near.
If they do have Whedon it will eventually turn into a heartbreaking misery run, for example did you know that Inara was apparently suffering from a terminal illness? And Wash's and Book's deaths would have occured anyway, Whedon admits the script he submitted for Serenity originally was everything he wanted to do, and he just trimmed it back from their, so wash and Book would not be saved, and this is a Whedon production, so you just know that some sort of ridiculously contrived coincidence would have got Simon about an episiode after he got together with Kaylee, just so River could go bezerk and kill everyone.
But without Whedon, it would be a fan show, and that would be even worse.