Yeah, most of the cool stuff is definitely more sequel material. As said we wont do any more expansions for V2 though so you will have to hold out for a V3 hopefully at some point. The old expansion model makes it much trickier to make smaller dlc/expansion packs as well.
V2 players are really loyal, which makes sense, there is really no other game quite like it out there, so while the game might not have been as popular as CK2, EU3, HOI3 etc i see lots of people playing it and being very visible on forums and reddit etc so doing a sequel would feel pretty safe. I think there are lots of things we could do to improve it much like EU4 and CK2 have done as well with much better interfaces and a good abstraction level without dumbing anything down. You can be sure it will contain a rethink on the market so we can get decent submarine mechanics. Submarines are awesome , but not really compatible with the current world market system.
Yeah I dont know that it fits that well and doing a switch mid-game will be confusing. Perhaps do it based on unit types so normal infantry fights as normal, tanks can perform movement is attack and artillery can not conquer but can support nearby attacks and bombard nearby provinces without movement. Just a random though.