|Ceci ne pas un checklist.|
If you accept the conceit that the PCs are going to get harder to kill as they gain experience, why limit yourself to thinking that the monsters they fight will stay the same, even relatively weak creatures like Goblins and Kobolds? Granted, any army will have a large contingent of unwashed zero-level conscripts regardless of race, but once they survive a few battles, won't they improve their survivability and capabilities?
I'm a big advocate of listening to the game world and learning what it needs. If you do decide to introduce a new race just to provide a new challenge for the PCs, where does that race fit in the game world? Who are they in conflict with and how have they survived this long? And can your players play a member of that race? Do they exist for a reason in the story of your world, or are they an after-market hack job to challenge your PCs?
|"The spear's just for show."|
Yes, it will take some work to get usable stats on these leveled-up monsters, but that effort pays off in a more consistent world for your players to explore. Plus, a high-level party will never be sure sure about engaging a small group of humanoids. And that's the way it should be, I think.
"Whaddaya mean the goblin gets a Wish?"