How quickly will mod support be introduced? I hope that everything except maybe of new mission types can be modded (if possible these too). Weapons, enemies, bosses, stats, abilities, factions (number of them too), density of interest points on the map, algorithm of random generation, size of map, possibility of creating maps which are not random (modders could do a lot of good quality maps), AI (especially AI), air and ground vehicles.
This way developers of PP could fix and work on the core of the game, improve its engine and fix bugs while modders would do most of the rest. Occasional expansions could add some new mechanics.
Such game would be worth subscription (for example of 50£ per year) as I imagine game which is too moddable would not need too many expansions or new versions.