I've seen Westonard's comments about expansions elsewhere on these forums, and while I didn't really agree a lot with his idea of them, I think they're growing on me.
Using his ideas as a springboard, I'm speculating the next expansions will be another two box deal, Order and Destruction. One of the minor races will be the spotlight, like how Dwarves and Vampires were for this first round. I speculate round two will be Elf heavy, with the Dark and Wood Elves being the highlight of their respective boxes. In addition to those, we'll see a single regiment of the other minor races (Skaven/Lizardmen), a single regiment for each core race, then a couple disks for Dwarves and Vampires since they just had a major release. So the order box might be something like:
Wood Elves: 3 regiments
Lizardmen: 1 regiment
Empire: 1 regiment
High Elf: 1 regiment
Dwarves: <1 regiment
Which could be mirrored in the Destruction box as:
Dark Elves: 3 regiments
Skaven: 1 regiment
Orcs: 1 regiment
Chaos: 1 regiment
Vampire Counts: <1 regiment
I find it very likely FFG will include at least new core race heroes in the next wave. I think the five races on each side is more than enough to handle right now, and I think Katsuyori hit the nail on the head that some races are very likely to be lumped in with others. I'd never expect to see Beastmen as anything other than Chaos disks, for example.
I don't think we'll ever see single race expansions, since it seems to be a better business model to release larger, more expensive expansions that nearly everyone will buy to get what they want out it, i.e. a strictly Orc player might not buy a Chaos-only expansion, but both Orc and Chaos players will buy an expansion that includes both races. I think many Diskwars players are a bit more open-minded about the races they have rather than someone in the miniatures game who typically concentrates on one race only, but splitting up the races makes it easier to create that divide, and ultimately might make the smaller expansions less profitable.
I don't claim to know the best way to go forward with expansions, but as a final thought, I think FFG will concentrate on the races established already before introducing more. I know as a semi-regular Diskwars player I'd be pretty bummed if their next wave of releases didn't bring much new to the table for the minor races (Dark/Wood Elves, Skaven, Lizardmen) since there is ultimately going to be that desire to play them as single-race armies.