Aliasalpha: Well basic design work, plans for the types of units and all the background type of stuff they didn't end up using. Probably made a lot of things that they wanted to add but never did until Winter Assault
the IG would be interesting to see in the game, not as much cover for a lot of troops. I probably won't get an expansion that only had multiplayer races though, not unless it was free of course
Whatever faction they add next, I just hope they don't throw balancing out of the window like they did with Winter Assault. When it was released it was only playable in single player mode. IG are difficult to balance. I'd imagine that the inclusion of Tyranids are only going to increase those difficulties.
As Blood Bowl proves*, converting the tabletop turn based rules into real time gaming is a difficult process and it's one area Relic haven't excelled at when it comes to retaining the balance.
As for a single player campaign, I hope they include one. One that's a little bit more focussed than the DOW expansions became. After all, not every race need be included in the campaign. Or they could produce mini campaigns that deal with certain races separately.
*According to reviews.
BladderOfDoom: Maybe they could have it some kind of IG kill team. Nothing but veterans and stormies would enable them to keep the nembers similar to how they where with the marines. Oh and sentinels, they would need sentinels.
I'd imagine they'd be more numerous, like Tyranids. The most basic lore correct Space Marine is infinitely more powerful than even a IG Karskin.
It's worth pointing out that the tabletop game isn't lore correct either. It was balanced to stop Space Marine players being left with about four units to use, compared to their opponent's massive army of Orks/Tyranids/Whatever.
So there's no reason why IG would have to be so few in number. Unless the engine just can't handle it or something.