The ability to have a fixed navigation
Yes please! This is quite an easy one!
I use the Mercedes theme and wish it had a fixed nav. For my work, I like the Mercedes layout better than the themes that currently do have fixed nav, so I don't want to change it. They should all have the option.
There are 2 themes with fixed navigation on top but I would be extremely keen on the ability of fixed navigation on the sidebar as well for viewers on larger screens.