In a way Aptom and the Booster Creature share a lot of similarities. Both absorb and combine with other creatures. The difference is that the Booster Creature creates horrifying amalgamations while Aptom is able to pick and chose elements from the zoanoids he absorbs to create a superior battle form.
I think the story plays out better with Aptom in the role since he was already a thinking being with his own personality and background. The Booster Creature had none of this except Sho's memories that it didn't seem to fully understand or comprehend. In the end it would have been developed into a dark reflection or counter part of Sho a role already being filled by the other guyvers. First Guyver II, then Guyver III until their resent battle and now that role will likely fall to Guyver II-F.