There's been a lot of things I've pondered as things to suggest, ask about, etc. Some involve potentially ambitious ideas like military uses for ambush, the gauntlet bug, etc. So instead I'll compile the ideas that might be simplest, or at least the smallest and least ambitious. And I'll run them down in my best estimate of least to most ambitious.
1:
Giant desert scorpions. It's been mentioned that Toady intends to bring them back, and possibly add normal and animal-people variants, but...why not re-implement them as they were, and save the task of adding variants for another time? Then again, the giant version would be different code-wise via copying tags from a future normal-sized scorpion, but I can't readily think of anything that would break from saving such changes for later, especially since saves will retain whatever version they were generated with in their raw files.
2: Regarding levers, and linking items to them. I kinda wish that, upon telling you that no mechanisms are available, that it would tell you whether you have only one of the requisite mechanisms versus having none available. Though for all I know, this could be a more complex task than it appears, despite it presumably running math on whether it has at least two mechs to spare at the moment.
4: Cannibalism and use of bodily materials from sentient creatures (see issue
http://www.bay12games.com/dwarves/mantisbt/view.php?id=9171). The only reason I'm including this on my list is because it USED to be a thing, though as Toady has stated he doesn't know what he changed to cause this, I suspect it is more ambitious a suggestion than expected. ;w;
5: Change MAX_EDGE of native silver and native copper to 10000. "no swords until we can pick mats" is no longer a concern since we CAN, well...pick mats, for stone short swords at least. :V
6: Kumis, kefir, etc. It is in fact possible to brew mildly alcoholic beverages from milk. I'll give my own use of it in my personal version of Kobold Camp as an example (which is why this is going in the simple half of the list, easier than hashing it out from scratch):
[REACTION:MAKE_KUMIS] kumis, kefir, various other related products
[NAME:brew fermented milk drink]
[BUILDING:STILL:CUSTOM_SHIFT_M]
[REAGENT:milk:150:LIQUID_MISC:NONE:NONE:NONE]
[UNROTTEN][HAS_MATERIAL_REACTION_PRODUCT:FERMENT_MAT]
[REAGENT:milk container:1:NONE:NONE:NONE:NONE]
[CONTAINS:milk]
[PRESERVE_REAGENT]
[DOES_NOT_DETERMINE_PRODUCT_AMOUNT]
[REAGENT:barrel/pot:1:NONE:NONE:NONE:NONE]
[EMPTY]
[FOOD_STORAGE_CONTAINER] barrel or any non-absorbing tool with FOOD_STORAGE
[PRESERVE_REAGENT]
[DOES_NOT_DETERMINE_PRODUCT_AMOUNT]
[PRODUCT:100:5:DRINK:NONE:GET_MATERIAL_FROM_REAGENT:milk:FERMENT_MAT]
[PRODUCT_TO_CONTAINER:barrel/pot]
[PRODUCT_DIMENSION:150]
[PRODUCT_TOKEN:drink]
[SKILL:BREWING]
And in the creature files for milkable creatures, using cattle as an example:
[USE_MATERIAL_TEMPLATE:MILK:MILK_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen cow's milk]
[STATE_ADJ:ALL_SOLID:frozen cow's milk]
[STATE_NAME:LIQUID:cow's milk]
[STATE_ADJ:LIQUID:cow's milk]
[STATE_NAME:GAS:boiling cow's milk]
[STATE_ADJ:GAS:boiling cow's milk]
[PREFIX:NONE]
[MATERIAL_REACTION_PRODUCT:FERMENT_MAT:LOCAL_CREATURE_MAT:FERMENT]
[USE_MATERIAL_TEMPLATE:FERMENT:CREATURE_ALCOHOL_TEMPLATE]
[STATE_NAME_ADJ:ALL_SOLID:frozen fermented cow's milk]
[STATE_NAME_ADJ:LIQUID:fermented cow's milk]
[STATE_NAME_ADJ:GAS:boiling fermented cow's milk]
[PREFIX:NONE]
Name could obviously be changed to something less wordy. :V
7: Sanity checks for the "pray" conversation. At a bare minimum, removing absurd options like the empty "favor/demand" submenu or asking them them if they work at a tavern. It might also be interesting for remaining options to be changed to more flavorful (like meditating on values instead of arguing them). Some options that currently don't make sense could always be reworked in if divine intervention and/or visions are fleshed out (i.e. asking for directions to a place).
And now, I'll go through the ideas that I have a reasonable suspicion would not be easily implemented.
1: Conquest of bandit camps (see issue
http://www.bay12games.com/dwarves/mantisbt/view.php?id=9558). This goes first on my list because this is the just barely within my definition of too ambitious to go on the prior list. Why, you may ask? Because I could've sworn you could be able to conquer bandit camps in the past. That means it used to be thing, but unlike the last entry on my list, it's been so fucking long ago that I'm not sure WHEN it got removed.
2: Conquest of other sites (see issue
http://www.bay12games.com/dwarves/mantisbt/view.php?id=8001). This falls under my "too ambitious" list because, as discussion on that bug report reveals, I puzzled out a very rough outline of every little thing that defines how a position (like a human lord) can be successfully conquered, but the OTHER half of the prerequisite is that the ruling group and position need to be linked to a site they can treat as their seat of power. It would be relatively feasible to allow temples, necromancer towers, and goblin spires to function as seats of power. Feasible at least, relative to extending this ability to elven and dwarven sites, which lack any structures that could conceivably be treated as a seat of power.
3: The gauntlet bug (see issue
http://www.bay12games.com/dwarves/mantisbt/view.php?id=6273). Basically, custom reactions that produce gauntlet/glove items are unusable due to lacking handedness. Two possible solutions: allow for custom reactions to apply handedness tags to products, or make unhanded gloves count as wearable on either hand. This one is the big one that'd pretty much make my day, but it's also one of those suggestions where I have no fucking idea how complex it would be to fix. >.<
4: Use of stealth in militia orders: And now we go back to genuine suggestions instead of bug fixes. Essentially, an optional toggle on the squad screen, that will make the ordered squad use stealth in the course of following their order. This would be simper to implement for the kill order, since civilian hunting operates along similar lines. For move orders though, things get muddled. I assume the order would be essentially to maintain position in stealth and only attack if spotted or if a hostile gets close enough, but code-wise I have no idea how it would be implemented.
5: Some fleshing out of an adventurer's attitude toward praying to their chosen diety, specifically how strong a need it is (I've yet to see it start as anything other than a slight need). One option might be to add a value that affects the strength of that need, potentially removing the need entirely if it's strongly negative.
5a: A potentially more complex alternative would be for that prayer need to be calculated based off the spheres that diety covers, and compared to any values that match up with that sphere. For example, a need to pray to a god of family might be stronger if that character values family.
6: Sanity checks for fortress-mode thoughts, based on what their entity is capabable of. Only really useful for modding, but...for example, lack of any clothing tokens for a given type could prevent thoughts from a lack of them (like not getting unhappy from going barefoot if the entity can't produce any footwear), potentially other examples if they can be thought up. This example is sadly the only FEASIBLE example that comes to mind off the top of my head, but it's also the easiest to work around (modding kobolds to have socks).