FWIW, extra complexity and complicated transmissions can be largely avoid with the "range extended EV" approach, where the gas engine is a generator producing electricity and doesn't drive the wheels directly.

That said, the best vehicle in that style that I can think of, the Chevy Volt, does actually allow the engine to drive the wheels and has two planetary gear/clutch systems. On the plus side, the gas engine is expected to need much less maintenance than a regular engine that's used in a much wider power range.