Hello !

 I have just a idea, and i want to share it ^^

 And if ships in space and fighter/bombers can interact ? And if the defender got more than just manpower, like buildings, DCA, Artillery ?

Attacker :

 Ship's bombardment => Damage defender's defence randomly, we can do this only if we have more ships ( logically )

 Fighters/Bombers strike => Damage specifics things, but can be counter by DCA

 Landing Capsule => Just your army who invade, but can be weakened by DCA

Defender :

DCA => For defend against bombers and capsules

Artillery => Explode opponent's infantry

Bunkers => Just protect a litte against orbital strike and can add a little bonus of hp to infantry ( if there are enough bunkers, logically )

With theses elements, variety of troops ( infantry, base of your army, tanks to shield your troops and air to strike ) and tactics ( attacker : blitz with tank in front of your army, air strike artillery etc etc etc, defender : prepare yourself with mines / trenches, or preventive attack... ), we can do more interesting invasions i think :p

 Tanks for reading, and sorry for my english, i do my best :x