In terms of worrying about the characters being too powerful, thus making an uninteresting story, that is a problem but not an insurmountable one. Just make their opposition that much stronger and invest more in character development.
Keep the combats as short and to the point as possible, if it can be helped. The worst thing that can happen is if the reader can hear the dice rolls behind the sentences. There are some R.A. Salvatore moments where the fights are really drawn out and felt very tedious to me.
The best aspect about a role-playing game in terms of storytelling material is that it has a living dynamic quality to it. Writers often fall into ruts and genres become predictable. Because you have several people contributing to the story that automatically creates a bit of enjoyable chaos that will keep the reader guessing. And because you have the dice contributing its own form of randomness then that adds even more. The problem for you as writer is going to have to be to trim the fat and to bend the truth here and there in order to provide the story with dramatic credibility. At the end of the day it has to work as a story on its own, so the truth of the game log be damned.
"Sojourn is never early, nor it is late-- it arrives precisely when it needs to." --Dan, ep. 334