No one can answer your question, I find it odd you're asking us that anyway. You need to ask yourself this, why do you want to move from MyBB? What is not offered on the software MyBB, or what is lacking that you feel necessary to run your forum? Once you've figured that out, why not then investigate the forum softwares that offer you that and then decide. No one can give you an answer when all of our needs are different, and all of our predicaments are not the same.
Furthermore, if you feel that moving to a different forum software will get you more members and or activity, I don't want to be harsh but that's not going to help you with that. Forums are all so similar and user interactive that they don't really mind the forum software if it is adequate, which in my opinion (and clearly in yours since you've used it for so long), MyBB is.