@Osmosis: yes, 95% of 1.153 mods do work for 1.154 EXCEPT the 5% - those using Warband Script Extender (WSE), so no Battle Time (Coop-ish mod that lets you host your SP battles to MP), or other mods that state explicitly they use WSE. Diplomacy should work. Try Brytenwalda too, if you haven't, it is BIIIG historical mod that uses Diplomacy and many others.
Also, the Steam key works for earlier versions, you might not even need to use it though - I was downgrading to 1.13-something and it detected my key automatically - but if you do, you need to convert it to proper format - check Taleworlds forum for the converter.
@Ozy: Hmm, been some time since I last played Nord Invasion... it's entirely unlike Vikingr (not to say either of the two is uncool), it's a Invasion gamemode - players vs. hordes and hordes of AI, similar to Calradia Slave Rebellion (though CSR sucks) and Full Invasion series. From my experiences NI is the best Invasion mod.
@Nistenf: Depens heavily on which game you have: for original M&B I'd recommend Europe 1806 (Napoleonic period mod with very nicely fleshed out civilian life features), Solid & Shade (be a necromancer! Craft eeeeevil items for stat bonuses! Do stuff FOR TEH LULZ!). TLD has a lot of good rep, but haven't played it myself.
There is also the 1866 mod, a damn fine Western mod with all things you'd expect: mesas, stagecoach robberies, bank robberies, US government, Mexico, Apaches and Comanches fighting a war while several lawmen/outlaws groups have wars of their own, robbing people at gunpoint in towns, shootouts at noon and Confederate rebels, Indian raiders and Mexican bandits added to the mix.
For Warband: Brytenwalda, as I already said, Rigale for more dwarven gameplay, Prophecy of Pendor for more fantasy flavor (yet of high quality), 1257 for more mid-mediaeval historical Europe, Nord Invasion for coop fun (well, fun when you stop being sucky 0/1st-tier butterknife wielder).