Better multiplayer, better integration of modloading into the actual game screen and better integration of modding into multiplayer would all be great and would probably boost mod uptake and satisfaction among your most vocal customers.
As far as mods played, I've been bouncing between AGOT, CK2+ and PB for the past month or two.
AGOT is sort of entertaining because of the largely similar sized states whacking into one another, but has SERIOUS issues with event spam, plot overload, overbuild of systems and in general a severe case of kitchen sinkism. Plus way too many features that don't work, or aren't worth the work they require to get them to work. I can't say I'd recommend it to anyone in the current state, but on the flip side I did nearly pee my pants at how hilarious it was to run through the enemy flambeing everything as Aegon. Plot spam can easily crash the game though, so be careful what you wish for when playing it.
CK2+ works reasonably well and has a number of usability elements I find dandy. But it's also got a lot of special case stuff and hidden modifiers I don't particularly care for, with the province prosperity thing chief among them (tax makes it worse over time, economic research makes it better). It's ok. I like the faction system, but I don't like how opaque it is and how few options I have for interacting with the various factions. I also dislike how difficult it is to form a separatist faction of Italians without picking up tons of random minor German states, or of aiding other Italian states in escaping from the HRE after I've accomplished that feat. I'd absolutely recommend it as something worth trying, but I don't tend to use it for play sessions or MP due to the number of counterintuitive or unexplained stuff I've run into. It does have a lot of opportunities for MP, so it can be fun in a SP experience.
PB is the most limited of the three I've played in terms of changes to the game, but by far the best polished. Balance might not always be spot on, but everything included actually works as advertised. Plus the crown and demense law system is the best thought out. I wouldn't mind a few more systems getting added, but for the most part it's an excellent experience with a lot going for it. This is the mod I use for MP, I recommend everyone try it if only to see what it does.
I also tried that mod that generally improves portraits and clothing, but it also slows down my system to a crawl.
