On the topic of why certain rules were or weren't in place for Season 3 and 4... Odd talked to me about whether or not to restrict the number of posts before starting Season 3. I advocated against restriction on posting with the idea that limiting the amount of roleplay is a bad idea and would be a big wall for people enthusiastic for contribution. During Season 3 I noticed (or rather, this is what I recall), that the roleplay was dominated by a few people who posted a lot. Seeing other people contribute was rare and because of this I figured that it it were these large amounts of content from few people that kept other people from posting at all.
I think Season 4 proved the contrary in that the active core were the only people prone to posting to begin with. So I'll be the first to admit that when I insisted on the post limit for Season 4 I made a mistake. And on this topic, I have not spoken to Odd in some time, and certainly not about X-COM, so I have no idea if he has thought about creating another season.
I'd also like to put this out there, considering the discussion regarding the connection between gameplay and roleplay (or rather TTRPGs and storytelling). The guide for the TTRPG Stars Without Number aptly says: "... you and the others in your gaming group should give a little thought to making characters that work well together. Grim loners make good book protagonists, but they don’t survive well when there’s no author around to bail them out." This captures quite well the difference between the way characters work in stories and the way it works in roleplay. Obviously there is a very big difference between playing Dungeons and Dragons over writing for a Season in X-COM RP, but part of it still holds.
Planning for character development is hard when you have no control over other characters or even the well-being of your own character during a mission. I am by no means a role playing expert, but I tend to see the creation of a lot of NPCs in your RPs to get your character needs to do something. When I read that it is not quite as interesting (from a role play perspective) as when the characters you interact with to try and achieve that goal are other players. Having your character convince an NPC to do something, or overpowering and NPC is easy, which takes away the thrill. I'd love to see collaborations that aren't always two characters playing equal roles, if at all possible. Replace an NPC with a PC for a minor role in your post. But this is all merely suggestions, do with it what you will.