Well like trig said, even in full gear and high levels you still struggle against massive orc warriors/berserker parties.
I hope you guys are also going to consider implementing bosses like dragons for example.
Where you have to find and fight a dragon in its cave it will be able to use fire breath attacks which can damage multiple targets
maybe you can get in some other quest a “dragonfireshield” for protection or whatever :D
Things like these would give additional playtime of course after you got the invasion thing going your planning.