OMG did you just invent factions controlling tiles or did I? Or is it under raps and we need to be shut down because we obviously know too much?!
Factions are coming soon, the bit about armies is what I yearn for...
I've seen threads by other players about what changes need to be made to armies. Having each army locked to a tile (like a 2nd pop) and attack tile by tile unless you bring enough to blanket the surface.The actual pop might act as an additional militia for the defender if happy enough. Assuming that POP didn't turn into a freedom fighter when the rebellion started. Militarized pops after claiming their tile will travel to adjacent enemy tiles and try to claim it too. There will be waves in terms of troops waiting to land on a tile if a battle is going on. As soon as one becomes free they land. If the defense is strong and maybe certain buildings can add adjacency bonuses to defenders your initial invasion troops might die, freeing up the tile to land on, and letting a next army start fighting right where the last one left off. If an invading force were to get a foothold on the planet it would allow for the invasion to progress much faster assuming the invader has an infinite amount of troops waiting in orbit to land. In other words a defending army on a tile would have to defend against an army from the sky and nearby tiles attacking it.
For faction rebellions this would be very interesting. Nobody leaves a defending force from what I've read, and the militia defense is already tied to the # of POPs. So if a rebellion starts on a world like this the rebels will immediately start trying to capture nearby tiles because nothing is on them to engage battle. The POPs will move to the next as a militarized POP, now if the POP on this tile likes your government enough it will try to fight off the rebel army. If they don't like you but aren't rebelling they will let the army claim it and continue on their merry way. Perhaps events tied to rebellion strength can add damage to these rebel armies and events available to you decreases the damage. Other factors like the # of POPS belonging to the faction on the planet when independence starts and rebel armies ganging up on POPs turned militia will offer balanced chances of rebellions on undefended worlds succeeding or failing.
Think of the possibilities, well placed forts staffed with clone troopers holding off invasion by weaker troops for years. Race riots between neighboring xenophobes and aliens that can only be prevented by staffing army pops ontop. Certain attachments or maybe traditions or ethics or something can give POPs a flat happiness bonus if there's a military pop on top of them (or behind, or in the corner, however it's represented). Now imagine if military POPs had their own factions and ethos like a regular POP. You would have to lose a POP to make a unit. With some exception being robots, or clones, or xenomorphs. Just imagine what added dimension of utility is added to each type of troop.
Clones can be pumped out quick and cheap if done this way. The bulk of them would have to be stationed in space. But you'd be able to have the luxury of having a massive army. They have seriously decreased ethics divergence and chance of joining factions. If in the new update POPs immediately join appropriate factions based on their ethos then clones will start with no ethos and when they rarely change ethos they would join a faction, putting them at risk at possibly revolting if the faction does.
Robots always run the risk of becoming sentient when synth, shifting heavily to egalitarian (if they're servants), and starting a war to join the machine consciousness triggering the crisis if it hasn't already. They would have a unique faction who's goal is to join up with the machine consciousnesses, unless robots are treated equal then they revolt in the typical fashion to everyone else. If robots were to revolt they'd form a strong rebel army POP. Can be mass produced like clones but costs way more and are stronger, should be whatever it takes to make a regular robotic POP.
Xenomorphs can also be mass produced, don't require spending an existing POP like the last two but has a flat chance of breaking loose and killing the POP it is stationed with before being quarantined and burnt. Making them the worst planet defender.
Gene warriors would require the POP you are turning into an army like a defensive, assault, Slave, psi, and elite guard army. A turned POP would have the same ethics/faction it had before, but it might start shifting now that it is in an army (probably toward militarist, maybe xenophobe if it's fighting a lot of aliens). Gene warriors if they revolt and settle down will have an added very strong trait if they don't already. Slave armies require enslaved POPs.
The only problem I see is how to address the fact that so much in the game is based on the number of your POPs and how you move between planets. What I described would come at odds with that. How would the game know that some military units count towards your total count while others don't? Also what about resettlement and what happens when you disband an army that is a POP?
My imagination can run wild for hours but the fundamental mechanics of the game prevents everything from being realized.