I don't see tanking as too much of a problem in league, cup and champions. It's a reflection of what happens in the real world when a team gets relegated. The relegated team will usually be stronger than the teams in the division they fall into, but this doesn't automatically guarantee success.
However where I think tanking will become an issue just as big as the loan exploit for rest packs is in associations.
Imagine 3 managers running 2 teams each, all in the same association. Each manager uses their 2nd team ONLY to boost their associations chances of success. Build up their 2nd teams by tanking for 2 or 3 seasons and they have 7 and 8 star players to spare.
10 seasons in platinum nets 1000 tokens plus around 30 per season. Total 1300 tokens.
10 seasons farming rests would net 7-9000 rest packs.
After 10 seasons of tanking, but with old 8 star players they can go straight to the scout list and buy a new squad. And have plenty of rest packs to train them up.
Meanwhile each of their 1st teams reap the rewards of being boosted by 100 tokens a season through staying in the platinum division of associations.
The new exploit?