this surely must be a feature request. must alse be far easier for the cg guys to create than kilometres of 3d world!
bkool has velodrome practice. The rest of the program is no where close to Zwift however.
how many riders on one velodrome? 
Software would only show a certain amount of riders. You could sign up to it in advance
Would be nice as a turn off from Wattopia into a Velodrome. Can just ride off normal route and enjoy the drome for a while or exit if not. Least this way it would give the option for those who want to use it also.
Even just yourself riding pacing for ftp etc.
Could go the same way as online gaming where you create an avert private or public and people can join. The idea of going round a velodrome with hundreds of people woukd be nonsense in my opinion.