I don’t know that these stats are 100% accurate, but looking at the field guide, there are roughly 10 park locked creatures and 22 arena locked creatures. The number seems to go up with each release. Why not incorporate a new mechanic where there’s no more than one locked creature per arena? As new ones are released, move some formerly arena locked creatures into the parks and move a park creature into the wild as a global or area spawn?
The more wild creatures there are, the harder hunting becomes. They could rotate the creatures or put some of them into daily migrations though.