Congrats on the promotion!!
The first is having decent ability to teach a class. I think being able to help others understand the art is an important part of brown belt. So at the very least I think it's important to be able to sub a class and do a good job, and if someone has a stronger interest in teaching, then I would encourage them to work on it at this point.
In terms of performance/game/style. A big thing I thought about was the earliest stuff I learned. I had thoughts like, "Okay. Black belt isn't that far. I can't be a black belt that's not able to do a decent mounted cross choke or have a a shitty closed guard." I didn't want to be able to pass fancy open guards or finish heel hooks but not be able to hold/sweep/sub someone in closed guard. So going back to stuff I learned early on has been a big thing at brown belt. In fact the keylock/Americana has become one of my better subs recently. Shouts to
@Calibur
I also think being able to start on the feet is important. Even if you aren't good at offensive wrestling or Judo, having some defensive wrestling, being comfortable on the feet, or at least being able to pull guard effectively from starting on the feet is important IMO.You may find yourself visiting another gym and from personal experience, it sucks to be a brown belt at another gym where they start standing and you have zero wrestling.
This last thing may sound TMAish but I think off the mat it's important to try and conduct yourself in a respectable manner. Nothing overly serious. But if someone is a brown belt and starting fights, or getting arrested, or unable to manage their life, then I think they should try to make an effort to get it together.