I very much doubt we'll get a DM mode. a) It received very little use in D:OS2, b) the effort required would be better spent elsewhere and c) the engine seems a lot more complex now, further reducing the user friendliness and increasing the complexity required to craft a DM mode that works. While I'm a frequent DM'er, I prefer this game to be super focused on delivering good stories, not half-assing a DM mode that won't be used. For module creation and online play, NWN is still king and I doubt it'll be knocked off it's throne anytime soon.
I expect the first DLC's will be typical WOTC type content: new races, classes, options, and so on. I'm all for it as long as it's not priced too high. It's a staple of WOTC D&D and honestly how they make money. More content is always good.
I imagine that they've either planned for a story DLC to give us level 12-20, or just straight up work on a Baldurs Gate 4 that comes in another 5 years and takes us from 12 - 20. Maybe they wanted two games (1-10 + 11-20), but then decided to just do one game + major DLC. Time will tell.
What would in my opinion be great, but probably not everyone's cup of tea, is DLC's which add more companions. Minsc and Jaheira might be good companions, but so far I'm not thrilled by the current party members at all. Perhaps they can bring in some new writing talent that doesn't write ridiculous characters and they'll be available as DLC's. I doubt it, but I'd be more than willing to pay for new companions with good (non-ridiculous) stories and a more grounded and pleasant disposition.