It would be awesome if we had an item that made a pet stay at a certain stage/age permanently, or at least for a pretty long time (longer than fifteen days). If it had permanent effects, it could be a one-time use credit shop item, and if it was temporary then it should have multiple uses. I suppose it should be priced similarly to how the Regression Ray was, if not higher. Personally I'm quite fond of some pets at certain stages while not at others. Being able to keep them at the stage I like best would be great.