I think that most of us would be happy with a "modular" approach, i.e. release first a major bug/fix and improvements pack for the existing stuff, then release progressively more stuff in new packs, adding new player controllable units if you wish.
I would happily pay for the different upgrade packs if they follow this simple guidelines:
-A first major upgrade pack as base for all future add-ons
-Strict bug fixes and code improvements released for free separately from new units&missions. This should be considered exclusively as the normal support to the user.
-Per pay addition of maps, AI units, player units, missions, campaigns in thematic expansion packs.
-The upgrade packs should be done in a manner -if possible- that the user does not need one to enable other previous packs to play the stock game+1st upgrade release. For example, one pack adds a bunch of AI units, but the player control of them with specific new interiors is a separated pack. Or a new pack adds a set of AI units and comes with revised existing missions that now add them (F.e. new artillery support, etc.). This would also give a nice variation to the existing scripted missions, because playing them with the new units added would be different.
In all, this approach would allow the user to "customize" his game expansion to a certain extent and better manage the money he is willing to spend
