So I just retrieved all of the Dragon Priest masks. Took some time, but I did it. Slightly lackluster reward though, but I'm curious... after defeating all of them, is there a secret dragon priest I can only fight, after retrieving the final mask?

No. After getting Konahrik there are no other Dragon Priest bosses. There are many other Dragon Priests, however they have no masks.

I personally think the Masks are fantastic. Not only do they have powerful enchantments but they also have the highest armor ratings of all helmets.

Don't get me wrong. For a long time, I used Krosis's mask. I'm just feeling a little lackluster after all that. Guess I just hoped for some epic battle or something after.

I was hoping for some epic battle as well, but I think Konahrik is amazing.

The last one (immune from diseases and poisons) is not useful for me as I'm already immune from poison since the beginning of the game. As Redgard I have never been poisoned. Once or two I got the message that I have been poisoned but it doesn't show in the magic effects list. That's why I choose this character:-))). Then I found an amulet with disease immunity... I only use the mask having waterbreathing and wear it only when I need to explore underwater.

You're referring to Hevnoraak. Konahrik is the final mask.

where can i meet non named dragon priests? in all my time i have never encountered one.

They're level 50+ boss encounters I believe.