Honestly I'd personally enjoy creating some neutral antagonistic forces to the world such as a pseudo-sentient/haunted forest dividing two major towns/kingdoms or like a group of rogues that travel the countryside. Maybe even like drake colonies along certain mountain regions or giant sentient insects. Something to add to the fantasy and make the world feel more alive outside of the main plot. That and like... in LoTR they befriend the trees who would be otherwise neutral, so these neutral forces can be befriended or antagonized towards other currently existing factions :3

[s]Personally I'd love a race of bug-like creatures with a Queen that has yet to provide support for other factions as of yet.[/s]