I would see a Profile Cover added to MYBB 1.9
This should probably remain a plugin.
Considering Google gives a lot of importance to page loading speed, it is better to have fewer images as possible. But I won't mind if there is an option to upload a cover instead of a CSS background. Either in the core or as a plugin.
I have said this multiple times and probably has already been decided, but being able to create custom file profile fields would be far better and useful for more boards.