I think trade routes are a great idea. Trade was obviously really important historically and it's nice that Civ V and Civ VI let the player control it instead of just representing it abstractly. I also think trade routes creating roads is cute (though I wish builders could make roads as well.) But there are two huge problems with trade as currently implemented.
First, it's a micromanagement nightmare. Spamming trade routes is clearly a great strategy and constantly refreshing 20+ trade routes in the late game just sucks. They need to make these auto-refresh easily. Or maybe have them not expire at all; give the player the opportunity to manually change them after a minimum amount of time (say 25 turns). If the player doesn't manually change the route, though, have it go on indefinitely.
Second, I hate that internal trade routes are more powerful than external ones. It should be the opposite! Ever since the Civ series got rid of tech trading, there hasn't been nearly enough incentive to play nice with the AI. Why bother? What do they really have to offer you? Trade routes are a great way to fill that void. Imagine if having strong trade partners was the key to thriving economy. That would be historically accurate and make the diplomatic side of the game way richer! I think external trade routes should be generally stronger than internal ones. External trade routes to declared allies should be even stronger. Internal trade routes can still have a role; they should be useful for establishing internal road connections and for helping to get new cities off the ground with a bit of extra food and production. But in the long run, empires who establish foreign trade should outdo those who rely only on internal trade--I think that is both good gameplay and good history! Please rebalance this, Firaxis. This is an enormous opportunity to transform the diplomatic part of the game for the better.
Also, foreign trade routes should always benefit both sides. It's just bizarre that in many cases the recipient of a foreign trade route gets nothing. Insofar as this system is trying to simulate real economics, that makes zero sense. Moreover, the diplomatic side of the game would be richer if you actually wanted other nations to send trade routes to you.