Well, one solution to getting more combat is to always wear a Legion armor.

Again, I don't think that the amount of enemies is the problem.
It's that when you find them, most of them are to mid-level enemies.
New Vegas has no level scaling or random encounters at all so combat becomes predictable, and the only way to make it harder is to gimp oneself.
Anyway, Enclave isn't really the solution to everything either.
What should be done is to amp up Khans, Powder Gangers, Scorpions, Escaped Boomers, Kings and Van Graff/Crimson Caravan/Gun Runner and give them patrols around the wastes.
Have a certain % chance that a group of these can spawn in a lot of areas with equipment that gets better as the player levels up.
Have them all send assassins/mercenaries after you if you screw them over.
This could easily fix the combat problem.
Anyway, Enclave isn't really the solution.
The problem is that the game is too static and that the enemies you do get all have lore-friendly equipment.
What I'd like is for hardcoe Mode or Very Hard to amp up their equipment and they could be just as dangerous if not more dangerous than Enclave was in FO3.
Enclave just seem to be a better choice because they used PA and energy weapons.
It seems that you're forgetting that their energy weapons were... Well.. Crap... And that due to the Damage Resistance system their PA was also pretty crap too.
So Enclave in FO3 is in no way the be all end all enemy.
So in a way, level scaling, while not very lore-friendly or logical at times, is really needed in New Vegas.