Is there a mod that after becoming Grandmaster adds more fights (other than the creature fights you can do once a week)? Like maybe another series of arena quests?

I don't know, but search the web for "Waalx Realswords" and download his Arena mod from his forum. Not sure how much he adds, exactly, but what is there is certainly intensified and elaborated greatly.

...The Arena was a bit sad, wasn't it? Blew through the whole thing in quick succession, met no resistance worth mentioning, am now addressed by many as the "Arena Grand Champion". I don't really feel I earned it.

Agreed. I'm playing through oblivion again (it's been awhile) and the arena is very fun but it's over very quickly. That being said, if someone wanted to add more matches I'd gladly let them use the armor sets from http://www.tesnexus.com/downloads/file.php?id=28245 to spice it up.

Fighting two http://www.tesnexus.com/downloads/images/28245-2-1258779126.jpg could be cool. Or a http://www.tesnexus.com/downloads/images/28245-4-1258743923.jpg. Or for the final match three warriors decked out to look like http://www.tesnexus.com/downloads/images/28245-4-1258411531.jpg.

Ohhh ... http://www.tesnexus.com/downloads/images/28245-4-1258778165.jpg. :hubbahubba: Maybe somehow make the requirement you have to fight these freaks at night.

Hi, my favourite arena so far is the one in the mod called Bartholm. You can fight whenever you like and the opponents are always different.

http://www.tesnexus.com/downloads/file.php?id=5022

You might also be interested in http://tesnexus.com/downloads/file.php?id=18113 or http://tesnexus.com/downloads/file.php?id=27254