We currently have this which works.
https://meta.discourse.org/c/support/none
The feature request here is for:
Default List Filter:
[all topics] (default)
[no sub categories]
Something like that.
Seems reasonable to me, sort of doable in a theme component but it is somewhat tricky cause you need to muck with ember routing and get a double request which is not ideal.
@codinghorror should I slot this? Has popped up often enough.