Some metal type monsters are designed after artificial technology. These include the evolution lines for "nut", "pythwire", "grimachin", "botbot", "pigabyte", and "hydrone", just to name a few. Perhaps there could be a way to "craft" these from materials gathered in a dungeon or dropped by some of these species. To keep it interesting, it might be expensive or risky, possibly requiring the player to sacrifice certain evolved species of monster to have a chance of working. And if the crafting process is unsuccessful, the sacrificed monster could be permanently damaged. It could even die and "haunt" the player in battle until all party members faint, then move on.
My idea might have gotten a bit dark towards the end of that paragraph, but this isn't that proprietary series.
I like this idea! Hydrone could be manufactured out of found items. Pigabyte could be made out of found items plus a Cochini, if you wanted a sacrifice (since Cochini is also a pig).
And this could be a cool alternative to Fossils in the proprietary series, which were also items that could be turned into monsters.
Being haunted could be fun too - perhaps there'd just be a ghost monster that would represent any failed sacrifice, that would show up and battle you at inopportune times.