I am also concerned about 5's for arena. There will have to be a refocus or shift I think for bg's and the era of rbg's.
every evening there is always teams playing 5's but just not enough. It seems like 10 man rbg's are the sweet spot. For pve and pvp, and definitely agree it's easier to balance in that situation.

As far as class comps go I always love to revise and tune. I think a combat rogue could work well since they are less dependent on positioning and requiring stealth.