I used to work in a book store at a large train station and one day an American comes in. He had a gift with him for whoever he was visiting and he wanted it gift wrapped. I said "no problem", wrapped the gift and wished him a nice day. He gave me 10 euro.
At first I didn't understand what he was doing and asked if he needed change.

I was a poor student, so I took it. Don't think we even had a policy on accepting tips.
In restaurants in France it can happen you cannot leave an electronic tip. Their systems don't allow them to add a tip. If you want to tip, you have to tip cash, which I usually do not have with me. Last time I had this was in one of the Disney Village restaurants.
Yes, it is a no

My first suggestion would be a positive review, because that brings in new customers and more revenue.
Second would be snacks/candy from your destination.
I would never bring back anyone, including friends and family, a miniature eifel tower when visiting Paris. Because they weren't there, they have no memories of that place. And unless something exceptional happened when booking your trip, your TA will probably forget who the gift was from.
But let's go back to the topic. Let's say in the future it is custom in the US to tip/gift your TA, what would it be? A percentage of your spend, a fixed amount, if it is a gift, what should be the value?