5 votes allowed...
I haven't included 'total overhaul' mods as a choice because those will take quite some time.
And of course, we all want to see a few of those.
I personally find such inquiry extremely useless, we have to see what will be actually featured in the game up to 6 months post-release.
Let's suppose the game releases as 1.8 current state, no feature with pending fixes for stupid little details, than there's no priority for mods, you simply gotta download a mega compilation to fix the game because it's extensively broken with missing basics.
Ok, past that point we can be more assertive as to what the game needs to "feel" better, than it has varying degrees of preferences. Anyway, let's talk about mods:
There are mods that flush out mini-features that I find essential but the mod's core objective's ultimately crap for me, either because it works wonky, or because the core of it is straight bad. So far I didn't have a mod that I can recall that of which I'd consider "bad core feature" (at least from the ones I use), but I do have wonky ones that I've kept because of mini-features and quality of life side-effects, the most notorious being Fourberie - I particularly find that mod very creative but I don't really like how it plays out, it's repetitive, packed with fourth-wall breaking glitches and has a lot of screen-staring into it's core loop system, I hate staring at menus/screen like a fool... Yet, it has both quality of life with alternatives for leveling roguery and getting some income while you grind for more important stuff, and the most key mini-feature it packs that I can't live without (becaues I def can live without the "quality of life" it provides) is actually the ability to recruit minor faction troops....
Now, there's a mod that seems to be considered "essential" by many that of which fixes stuff but also breaks stuff at the same time, that being Diplomacy. If we are to compare, Diplomacy for WB did correct a lot of missing behaviour into the game with tons of caveats that should be in it officially (yet TW dropped the ball there which seemed forgivable at the time, but today seeing what they did with BL I simply refrained from forgiving it), it added quality of life and made for a very much needed balance of which base-game WB lacked (M&B and WB were sort of atrocious in some balancing regards, specially for late-game kingdom management). In BL, diplomacy bends the entire base-game logic rules for the sake of "only the author knows", AI behaviour becomes more erratic with Diplomacy (as in constant war votes - no weighting added for AI decision making - war scores remain bonkers non-sense where you either spam raids or wipe out almost the entire enemy armies, otherwise you are always "in debt" being forced to pay tributes no matter if you handed their asses to them or not, because territory capture's really ilogical in that regard). If diplomacy either had a extensive fix for AI and respected borders, I'd say it's quintessential, since that isn't the case I just find it half-desireable and stick to it because of, again, quality of life additions.
This same logic of criticism can be applied to many mods, as such it's also hard to judge which mods I really want or not, in cases like these 2 I just use them, but I don't really like them...
Than we have other mods that touch on other barriers of which I find necessary because TW ppl is not good at gameplay balancing imo (my opinion, though I do believe more ppl share the same opinion even though they don't openly state it). That ranges from everything to everything... From combat to passive income, to gameplay loops, leveling speed, gameplay utilities availability, etc... - From the top of my head the moist aggravating issues are early-game balance, absolute progression (crap balancing), level up gains, item gating, etc... For this general balancing I do find myself very inclined to maybe, just maybe, come back into modding games just to fix BL from a Game Designer PoV regarding it's challenge/reward curves, but I also have some lack of desire of doing so because the game lacks any late-game content, meaning you have to either handicap yourself during playthrough to get the most out of it, or you'll hit progression cap relatively fast -> as is is much worse than that, though, because the most rewarding and efficient hidden mechanics of the game force us to endless grinding combined with pointlessly over-extending a campaign just so we can rack in the decent rewards... Translating part of what I'm talking about, kids will always have a much more extensive access to focus and attribute points than any created character or wanderer. That's time-gating and makes for crap experience, nobody manages to get their kids grown in sandbox before the game's virtually over, that means the calc for focus and attributes on level up's crap... Than we have the comparison to actual AI lords and total pt availability, and they are all well above our created PC, at all times. When you see a noble with low skills in the encyclopedia, that just means they are low lvl, yet if you level them up they'll sky-rocket and have a considerable + on both, while we're stuck with a crap generated PC that can only get on par with them by delegating the PC role to the kids. This means that since I first played BL I simply can't stand not using XP adjusting mods and pt mods, feels cheaty sometimes, but even using them under toned down settings will still allow the game to eat up dozens of hours for you to reach a good build - which begs the question as to why are they shoehorning time-gating in the first place...
So that's that. If I'm to guess things that will probably continue to need modding to work decently, progression and probably their crafting are the first to come to mind. Also other crap quality assurance mini-nuissances are always good to mod out, like NPC banners and other stuff related to them (banners in the game are whacky, glitched, incomplete and sometimes even broken - kigdom banners always glitch when modified - tons of clan banners are totally whacky with misplaced parts - tons of symbols are not centered - there's no layering to build a creative banner - and kingdom level banner symbols are not even available to be picked by the player) - like the banner thingies, there are many other minor-glitches that need adjusting to make for a really rounded experienced, and these have been overlooked for so long during EA that I tend to believe we'll still need mods to fix them...
Anyway, those are just my thoughts, I also look forward to good overhauls that add RPG elements to the game, that'll be neat, specially considering that now we can have tons of in-scene features translating into having a really deep experience with the game - and I ain't thanking TW for providing a more "open" engine, I would if they actually did that themselves (added said depth).