GTFO... jk 
There's definitely going to be a mode aside from the Campaign for SP... Don't you worry your little head.
Hell there's even plans to try using Elemental to make RPG's of sorts. (Brad himself said something about tryin his hand at it.)
As far as I know for SP there's
1. Campaign mode (Supposedly plays kinda like an RPG, Don't get confused though, it's still a Strategy Game)
2. Free mode or Skirmish mode or Whatever... Just load up a random world map (Doesn't HAVE to be random) add your opponents, choose your side/sov and kick ass.
3. Modded in modes... RPG's, different Campaigns, etc...
I think that covers it.
I'm pretty sure on most of this, but If someone sees something I got wrong or can elaborate be my guest.