Forced induction revamp suggestions
The first thing I don't understand is the fact that turbos are only available from 1975 while the first production car was introduced in 1962.
SUPERCHARGERS
Roots
Twin-screw: An improvement over the old Roots type supercharger. Available from 2002.
Twin-charger: Combines a Roots-type supercharger for low revs and a turbocharger for high revs. First used in high-perfomance applications but now used in more economy oriented ones. Available from 1985.
TURBOCHARGERS
Single turbo
Twin parellel turbo: Twin identical turbos, reduced turbo lag.
Twin sequential turbo: Two different turbos, each optimized for a different rev range. Reduces turbo lag. Available from 1986
Twin-scroll turbo. Reduces turbo-lag and has overall improved perfomance
Variable Geometry turbo: The most advanced option. Comes at a high cost in both engineering and materials, but it's the most effective turbocharger. Available from 1988.
INTERCOOLER
Air-cooled
Water-cooled: Reduces turbo-lag. Available from 1988.
Water-injection: Skips the intercooler engineering process at expense of higher running costs. Was used in early turbocharged cars, but is still used in some aftermarket modifications and high perfomance cars (along with a regular intercooler).
There are some other forced induction options but I'm not sure if those have a chance to be added.
The G-type supercharger used by VW in the 90s was discountinied for no reason (that I know of).
Centrifugal superchargers, Eaton-type.
Quad turbos were developed for the GT90 concept car but are not used in the engine configurations we currently have (maybe with W engines in the future...)
Cheers
EDIT about the interface: I think that those could be easily fitted in the current interface. Just make the compresor, AR and turbine sliders double so we can choose the two different turbos when needed. Water-injection could be an option right below the intercooler tab.