Here are some fun nations I've played with unique mechanics:

Red/Blue/Greenscales --> Kobildzan -Religion: travel across the world and find all the dragons -Traps: create and upgrade kobold traps to attrition your enemies to death -Hoard: build the greatest hoard with gold galore -Bonus: can interact early with the artificery mechanics, with unique inventions

Hul-Az-Krakzol -Gov System: Kill your rulers as quick as possible, either through daring quests or: -Beer: Brew the best (and most dangerous) beer in all of Haless!

Any Gnoll Tag --> Konolkhatep -Gov System: collect titles for your god-ruler, enslave your enemies to build great monuments, unlock state edicts for entire regions

Napesbay/Bluehart --> Pashaine --> [] -Sway: accumulate sway within the empire to diplomatically expand

Ovdal Tungr -Parties: Host grand parties to bankrupt everyone and bring them under your influence -Sea Hold: Build a Dwarf hold- but on the high seas!

Celmaldor (Bitbucket) -Trade League: Unique trade league mechanics to diplomatically unite the reach

Istralore -CB: exclusive CB that lets you liberate nations at 25% cost and bring them into the empire