I love a lot of things in masterwork, and other things I never used really.
I don't personally care much for other playable races, never got around to trying them actually. So that's a low priority for me. It remains a game about dwarfs for me, so that's where I'll be playing most the time.
I did enjoy a great many of the additions, from the various workshops to the additional "tiers" of metals. I really enjoy small convenience things, like the brick maker for instance, and like some trade offs when doing the smelting. I can cold hammer in a pinch, throw it all in a giant cauldron for quick processing in the blast or invest a bit more to stretch some resources with the crucible. I think that's something great, and I'd love to see that kept and possibly expanded. We're industrious dwarffolk, I'm sure we have a variety of ways of working metals to suit our needs!
I really enjoy the added variety of plants and wildlife, with their various benefits. The steel and iron oak trees I thought were an incredibly cool feature, and the working of bone and blood to forge some grim but durable metal I really liked! This is perhaps a part where new "races" may be an option. Instead of getting humans, elves, kobolds and whatnot, more specialized dwarven civilizations that have their own thematic differences. A bit like warcraft has it's regular dwarves, the more nature attuned wildhammer (steel oaks restricted to them, some pets restricted, etc) and the more grim dark iron (blood steel restricted to them, golems or other artifice etc). (note: discard the names, I just used them for description of possible differences)
What I do enjoy most, and I REALLY like that aspect, is the added threat from various sources. Digging into a cave? Cool, now you get to fight a hive of giant ants! That's beyond cool, and each cave level being noticeably varied really did a lot for me. Similarly, I enjoy having more varied invaders. While I'm not overly fond of some types like the clockworks or such, they do add a lot by offering variety. Though personally I'd just be happy to have hordes of weaker invaders to slice through
Castes are a really cool system too. Specializing your dwarves is incredible. Though I think it may be okay if most dwarves are just generic dwarves (say -25% xp to everything), and specializing them was made convenient and accessible relatively early.
Definitely I would say what I want to see is a renewed focus on the core aspect, which is dwarves, in fortress mode. There can be various kinds of dwarves, which may differ physically when it comes to martial prowess and speed, or even things like tolerance to light and the likes. Or cannibalism or whatnot. Some of those civs may have access to things that others don't. More sturdy and cannibalistic dwarves that have a hard time getting mithril, but get into blood steel instead. Some more naturally attuned ones, who get minerals by planting trees/plants that drain the earth of iron/copper/whatnot and can then be harvested and processed (more labor intensive, multiple steps) but would be less limited by their surroundings and have a renewable sources of metal. The more dratitional dwarves we're used to, with a knack for smithing and getting those high-quality metals produced in good amount, like mithril and whatever trumps that, while being fond of trade. Another civ could rely a bit more on clockwork constructs, and be unique in that it uses gunpowder for a very solid ranged option, while golems or machines pin the enemy in place. There's lots of options in making the dwarf experience unique and varied, by just adding and removing a few options from different dwarves civs, and fiddling with their ethics a bit. There is imo no need to implement fully playable different kinds of races, which all takes a lot of time. Dwarves are such a solid foundation and could be morphed into varies branches that are different enough yet keep the core gameplay to make the experience fresh and unique for years to come!
Most importantly though should be a consideration for stability and performance. I really came to love generic materials, not just for performance purposes, but also for ease of use in many aspects. Sure, a little bit of flavor is lost, but the benefit is quite enormous!
Either way, I'm looking forward to what you have in store for us, and hopefully there will be adequate documentation for the mod when it comes out. I really enjoyed the manuals you made, and they've been most helpful many times.
Cheers and thanks for your renewed commitment to make an amazing game even more awesome.