Have the sub forums show on the main board like discourse has. As an option.
Main forum title
sub forum 1, sub forum 2, etc
One of the things that keeps cropping up when I discuss my site with people is that they don't know when the last post was when browsing on a tablet in Portrait.
There seems to be enough space to add it, it's just a matter of where.
A proposition that could work, would be to add it in place of when the topic was started, or after it.