It's not that I want to drop out, or wouldn't want back in. It's just that I'm trying to find a way to make it work with the numbers and factions we have and that seems like a very workable option. With an odd number of players the problem of how to arrange the challenges, and one player not getting a game is difficult to work around. It's not like I wouldn't be around, or contributing to things.
As to the fluff side of things ... chaos and 'nids both want to consume and transform the universe. While they wouldn't work together directly/communicatively, for the purposes of factions I think that they fit the best. Necrons there might be some debate but I lean in the same direction of reasoning, and all the moreso given the eons long animosities and wars with the eldar.