Themeforest is pretty easy, it's hard to find any pain points.
I have developed so many wordpress websites using envato themes. They have awesome support through forums, most creators will be available to communicate directly through mail.
Price is low, payment is easy and choice is plenty.
I think your challenge is not in finding pain points, but in achieving all these basics that buyers normally look for.
Sorry if I'm discouraging you. It's really great that you want to create a similar platform.
I pain point that I can come up with is that, there is is a setup charge for all themes.
Basically you have to pay double the amount of theme, if you need someone to install it for you. I know its additional work and hence be charged. But if you are starting out and you offer initial setup free of charge, lot of people might be interested in it.
You can also try to offer domain and hosting services along with theme, which makes yours an all in one platform. That will be unique!
Good luck with your venture mate.