Well, though Civ style tree would be the simplest to implement I'm going to fantasize here a bit: among loads of strategy games, I would say a mixture of Hears of Iron 3 and Stellaris would be best. In HoI3, along with research points "produced", there was an experience in relevant area as well, namely if you specialized in infantry you would do that techs faster- this is both historical and logical. Stellaris on the other hand has research buildings, but research costs are scaling how big your interstellar empire is, enabling you to play "tall". This is important as largest nation may not be always the most developed. Compare Luxemburg and India. This would let you play tall, and benefical for AI too. Also research takes simultaneously in three seperated fields, namely physics, sociological and engineering. Research costs of course should also be scaling with difficulty as well. So what I fantasize is a mixture of experience and research points. You cannot straight out make a fur factory if you have never produced fur before, but you general development and technological level still helps. While general techs can be researched, specific techs are unlocked by having relevant tech level and doing it.