I think it could be more important to look at your desired endgoal and rethink possibilities to reach this goal. What are we trying to achieve here (is everyone even on the same page about this?)
The main goal I see brought up is to stop the same small group of players from dominating the TG, and with this the general, economy for online play.
The thing is, there are some factors that tell me that changes to TG won't change this much or even at all.
Every one of these people probably have more playtime in a single season than a casual like me has over my entire median XL experience. They reach endgame faster, they farm more, they simply have more because they do it better and spend more time on it than the average casual. So with or without TG, they'll be running fauzt when I'm doing K3K so to speak. This means that likely they will hold most of the items that people would like to trade.
They know and help one another for favours. I'm single player because I dislike server ping and the multiplayer experience wasn't all that much for me. So I look for shit like a last malthael set piece for days or weeks, because, I have to find this myself. At the start of the season, TG or not, if one of these guys finds a great rune in their normal hellforge, it's very possible that they simply say to their buddy who can use it: "ok you can have it, as long as you give me <your first drop of item X> or <the first great rune you get>, ...
I can't make deals like that on SP and even if I were MP I couldn't because I don't spend the time ingame to get to know people like that.
So in general, changing the TG system will probably not have the desired effect, since 1. these people whose monopoly you are trying to break up actively keep this together (not with the very purpose of running a monopoly, but just because they can, and it's easy for them). And 2. they spend another form of currency that I and many others can't afford: time. More time spent equals more items, compound interest, etc etc.
To really make the economy fair here you would need a lockout system (only run dunc 3x a day), or equalize the item distribution via coding (the less time you spend ingame, the higher chance you have to find set/SU/SSU/SSSU for X amount of time based on how long ago you logged in or something).
I think the most important thing here is that you have to keep in mind that you can get away with giving a specific group people more and still keep everyone happy. But taking something away from a specific group will without a doubt rustle some jimmies.