A rotary engine is a large task, but i beleive it would open doors to people who just want to do the rotary haha. Also turbos. bigger turbo
I really don’t see the need for larger turbos (at the moment) because the game is about producing production cars not supercars. Just imo.
The size of turbo is limited by the actual space available, so once turbo gets compatible with another exhaust manifold the size might be able to increase.
The large task we’re more worried about finishing is designing cars and running a car company 
Perhaps as a patch after the game’s release?
Or maybe DLC?