What did you have in mind for the inventor?
It would be a unit somewhat analogous to the Storyteller line. I just notice that the Storyteller can build buildings. The inventor's primary mission is to 'unlock' units. I think that can only be done currently by him building the building that is a prereq for the unit. It would have to be a special building that cannot be built any other way.
This applies only to units that have a technological component that is neither explicitly a tech, nor a 'no-brainer' application of a tech. So the siege engines, artillery types, ancient rocketeer, catapult, even the crossbow - that sort of thing. (Several of Hydro's new bows sound like they might need inventing...) Most of them up to the Renaissance are attached to the Ancient Machinery, Ancient Ballistics, Alchemy, or Metallurgy techs. (Then in the Industrial and later eras, there are a lot more of them.)
In gameplay terms, it means a delay after the relevant tech before the unit can be produced, as well as an investment of hammers in building the unit. The unit would be unlocked by the 'breakthrough' technology, so he could not be pre-built before there was anything to invent

.
It's a similar mechanism to what happens with the unlocking of the Siege Weapons Workshop. The tech does not unlock the unit, it unlocks the building which unlocks several units. In this case, it will unlock the Inventor. The Inventor is the only way to unlock certain unit/s dependent on that tech. If there are multiple units available to unlock, one Inventor can only unlock one of them - it is up to the player which one.
In realism terms, it represents the fact that the discovery of gunpowder didn't turn overnight into artillery pieces. It needed to be found applications for, and adapted for them in often ingenious and counterintuitive ways.
Unique (eg. the Hwacha) and alt-timeline units would probably be exempt: in those cases the culture wonder - which has taken time to unlock and build - can substitute for the process of invention that this unit represents.