Except, as we've established, "instead of" the MX-5s, Skylines, RX-7s, S2000s, NSXs and whatever else, you'd get nothing at all.
If it could be in the game, it already is. Otherwise it's missing because it wasn't modelled (they didn't do it in time, or couldn't get a real example), wasn't licensed (the licence owners did not negotiate to the car's inclusion), wasn't chosen (PD simply didn't see a need for it) or wasn't known about (the guy making the "car encyclopaedia" somehow forgot it), in ascending order of requirement.
If PD didn't know about the car, telling them about it is great. Only they probably did know about it - they're trying to make a car encyclopaedia. If PD didn't want it in the game, telling them you want it is great. Only they probably do want it - they're trying to make a car encyclopaedia. If they couldn't get a licence for it or couldn't get a real one to model, telling them about it is superfluous.
Personally, I quite like having a 1.6 NA, 1.8 NA, 1.6 NB, 1.8 NB, 1.8 NB.5 and 2.0 NC MX-5 in the game. The fact they're replicated for Eunos Roadster/Mazda Roadster, Miata and MX-5 regions is... odd, but understandable if you know where the car list came from and why. The alternative to having them is having nothing.