What's up with these boring suggestions, anyway?
I'd suggest moving on to plants after letters, starting with a very small kind of plant right after version z - say, a version named after some sort of algae - then moving upwards in size as the versions increase, stopping by such wonderful version as version carrot, version tulip and version potato, then reaching even further and paying homage to lovely trees, such as the humble dwarf birch, the proud larch and finally ending somewhere around the mighty coastal redwood.
Then, it's a simple matter to move on to mushrooms, animals, and whatever else seems like a good idea at the time.
I'd like installing the pumpkin patch.
(Seconded?)