This is just an idea to very slightly tweak trade in our good game.
What if the trade benefit of a given trip increased as time goes by? This could be turned off by default in Campaigns, etc. but it may allow to break some very long team games, or to keep them dynamic. It could even be locked behind a market tech (which all civilisations would receive).
To get a sense of how small I think this could be, I am thinking +10%/hour in game time. But remember this is multiplicative (actually *1.1) so it could be lot after a very, very long time.