I like this idea. Both players set duel mod bo 3, during those games they lose their tcs like in a typical /duel mod, but after 3 wins a winner gets his and opponents tcs. Leave is a loss, spectating is only acceptable by agreement of both players or its unavailable at all, you can only pause the game during the duel, for example:
Siou vs Fake, duel bo3
/duel mod (amount of bo, in our example its 3) 20000 (TC)
Command fspecs both players and whenever both of them enter the game, duel starts. (Works like a casual duel mod)
Siou needs to brb, so he writes a command /pause but he needs his opponent's agreement, so his opponent also needs to write /pause
In case of 2-2 score both players can change a duel to bo4 by agreement and another command
After the duel, winner gets his and opponent's tcs
My English is bad but i hope you got my idea