In terms of DLC-thinking, if each DLC is one central idea, then I'm basing it off of games that I know and play namely Sims franchise and Cities Skylines
Education - super educated or unskilled workers, but since kids are not in various ages, I suppose limit to elementary and universities for Adults (new survivor arrivals) and specializations-focused so we can tie in to create more Specialists that way
Seasons - more weather systems other then Winter (and player can select the duration of each or select the ones they want or don't want). Fall, Summer, Spring. Forever Drought scenario? Once Bunker is build, use it as an actual bunker during Disasters...
Industry - more resources and choice of manual, or automation. There's always the extreme spectrum of each that a player can gravitate towards. And expand the Trade mechanism
Safety - for those that thrive on moar combat and conflict.
Logistics - wheelbarrows? And other transportation for cargo? Would make more sense if we have a bigger colony map, which I believe at the start of Early Access the map was much bigger IIRC. And would then make the much-requested minimap a necessity.
But this might be a sticky thing as most of us think Logistics should be improved in base game itself, not in a DLC
Basically to expand on the 3 ideologies further or even a chance to create more?
Health - moar Hospitals to treat diseases and nurses/doctors have specialisations to treat the different diseases, moddable diseases
Food ecosystem - Pets/Seeds - domesticated animals, beasts of burden, hostile animals/pests, more varieties in Farms, Ranches and therefore Recipes. Expand on the current resources. We have Cows and Pigs but no leather, bacon, etc?
Skills - with education, industry and food getting expanded, might it be more interesting to introduce this? and the super-skilled becomes Specialists.