How much of DFhack will be incorporated into / work with the new Steam version,though?
Nothing has been mentioned to be incorporated, which is to be expected, both because Toady generally doesn't mentioned planned details because they might not make it in, and because they haven't really reached the functionality improvement part to any extent yet (and it seems Toady doesn't look at what DFHack does much, and so might not even know if something is supported or not). I wouldn't expect much of DFHack's functionality to be included, though (a Sand indicator seems like a low hanging fruit, but we'll see [there are a number of bugs in the embark display info that may have a higher priority, though]). When it comes to DFHack I'd expect most of the functionality would be supported for the Premium version when DFHack catches up, but it will probably take longer than usual, as the new UI will at the least make the current DFHack UI functionality badly matched, and at worst would require a complete rework of the widgets from which the UI is built, and then rework of the functionality to use the reworked UI. Key bindings will also have to be reworked to a lesser or greater extent as DF reassigns keys.
Ok, then that's my big feedback :
Incorporate all the shit in the most popular mods into the Steam version. Those are the things players want. If there's a popular mod lots of people use that's adding, e.g., searchable sand,
be sure to add that. That's important.
People loading the game up on steam are not going to listen when you tell them "well actually you need to go install six mods to make it fun". They're just not gonna play, and then they won't recommend it to their friends, and then sales don't happen. Plus if one of those mods isn't maintained, which happens often, you've pinned the success of your game on a broken axle.
So that's my big suggestion: make a general pass through the most popular mods and be sure that the features those mods add are part of the Steam base game. Graphics is the obvious play here but if there's a mod everyone uses because it enables good seiges (e.g., Masterwork) or basic search features (Sand, etc.) include those also.
You can always make it a togglable option if you want but the options should be there in the game. All those modders have done all that work adding the features people want; capture that work and use it.