It all depends on what will Zoro do and will it reach the right people, like Morgans and World Government.
Considering that X-Drake is present and potentially some Cipher Pol too, as well as Wano incident being known all around the world, post Wano bounties should reflect the actual strength of the crew.
If Zoro spends most of the time fighting King, I assume his bounty will be greater than King's, at least.
If Zoro spends most of the time fighting against the Emperors, his bounty is harder to predict.
If he kills Kaido and that gets reported and reaches the right ears, his bounty could be very high.
The very minimum bounty should 1Bil, however, such a low amount makes no sense.
If his bounty becomes higher than King's it should be in the 1.5Bil range, minus 100mil, plus 200-300mil.
If he kills Kaido, it could perhaps go above 2Bil if the WG takes it seriously. My guess is 1.8Bil for now.