I don't think there's anything underpowered about Rogue 5 as long as you choose skills that are relevant to the campaign, and realize that you probably won't compete with the 2 druids (or other party members, most likely) in most combat situations.
This is just a sidenote, but in my opinion, it's critical to play a build that you enjoy NOW in a PBP, because it can be 1 or 2 years of RL-time before you get to level up in many games, and delayed gratification builds can seriously suck. I'm in a Faerun game that's almost finished after 3.5 years, and the characters have gone from 3rd level to 4.6ish (the BBEG is the fight immediately after the current one, so I'm hoping to level before that...). That's an extreme case, but it's something I always keep in mind.
Air Goblin might have some synergy with Stonedeath Assassin (a PrC previously mentioned by another poster), since it uses meld into stone as a tool for assassinations, and air goblins don't breathe (and stonedeath assassins get even more bonuses against those damned earth elementals...which is...flavorful, if useless), so if you can manage a burrow speed somehow then it could let you do sneak/death attacks as a mole.
You've got 2 feats to play with...what're ya gonna do with them?