So many good suggestions here!
For dub VAs I want to second the Critical Role folks... they fit more than one fandom, with a great crossover of VA work in anime and in gaming, and then of course representing table top gaming & D&D. I know it would probably be very hard to get them all, and that they always do GenCon which is around the same time but... it would be amazing!
If impossible to get the full Crit Role team, then atleast Laura Bailey and Travis Willingham, please and thank you!! Their panel a few years ago was one of the best I have been to and they are just great.
I want to second the request I saw earlier for Little Kuriboh and Team Four Star to come back!! Again, the Abridged Panel many years back was still one of the best Otakon panels I have experienced.
I would love for Chris Sabat, Sean Schemmel, and Kyle Hebert to return, but I know that is rare, and really this year would have been the year... but still, would love to see them!
I haven't really gone to any of the concerts in all my years attending so far... usually I opt for more time to do other things, since the concerts take up so much of the evening... Distant Worlds was close, but in the end I didn't want to spend all day in line for only one event. But, I saw some great suggestions here, and for any the following I WOULD probably skip everything else to see them: L'arc-en-ciel, Asian Kung Fu Generation, Momoiro Clover Z, Utada Hikaru, and Ayumi Hamasaki.