We had a few of us during the day run a bunch of tracks on my server today. Of the ones we ran id say Brands Hatch could easily be used for a league race.
Also one to keep any eye out for but not working in MP yet is the istanbul scratch made pre alfa. That track surface and textures just about put official tracks to shame. Amazing work by the guy and once this gets updated a bit will be one of the best tracks in AC I think and could become a regular league track.
We also tried Monaco (converted) ok but seems a little off, still ok to race seems a heavilly edited gp4 or ISI f1 track. Done well for the base but a scratch made track is in the works and I think looks much more promising but also needs lots of works still.
Tried a Barcelona track ok a little bumpy in corners though quite subtle but runs quite well and looks fairly good.
Tried Laguna Seca 2.2 just ok I think not great.
Also tried an Albert park conversion, was ok kerbs seem a bit off but not bad obvious codies conversion.
Still the only of those id say should definatly be considered for league use is brands hatch. The others id rather run any official track before using them.