Not personally fond of mystery card sets. But if you are going to create enough ATG cards for having a mystery game why not just add the required number of cards to ATG and then make mystery or non-mystery an option when creating a league?
So those who like mystery game can create/join a mystery league and those who do not can have access to all those extra cards. Everyone is happy.
If this were done it would also be nice if there was an option for doing a hybrid. This would be an option when creating a league for generating a custom player pool where for each player one year is randomly selected. Everyone sees the year and that year is only one available for that league.
While dreaming why not make the above available for all mystery games? Or merge them all. Then when generating an ATG mystery league you could select a decade subset if desired. Somewhat similar to how you can now select pre or post WW2.
Agree with any ATG set being limited to players with at least some number of years minimum length career. 10 does seem like a tidy round number though it could just as easily be 9 or 11 or some other number. Or maybe even have an option when creating a league to specify the career length to be included. Selecting 5 gives you every player while selecting a higher number eliminates those with shorter careers.