Civ needs more "rubber bands". Currently there are only tiny tech discounts and espionage which is limited to midgame, when runaways dont have anti-espionage buildings yet. But even if there was a powerful mechanic like tech diffusion (actually there is a mod by Afforess), that still was not enough.
My idea is investment - trade routes to backwards civs are more profitable than to same level ones for the owner, and the destination civ gets even more. E.g. trade route owner receives much gold, and his underdeveloped partner gets alot of production and faster tech acquisition. Developed nations could even wage wars for markets, if incoming trade routes were limited.