This could be a diplomacy skill:  wager.

Imagine a dialogue going something like this...

PRIPYAT:  Prepare to have your city demolished, [Metasoul].

METASOUL: I bet [500 gold] yer can't!

PRIPYAT:  You're on.

Players could choose not to bet, but then their opponent might get a morale bonus.  It's like calling your opponent chicken before a fight.

Winner gets the normal outcome they'd usually get, plus the wagered amount, which is deducted from the loser player.

Wagers would play out in the diplomacy screen, so players could wager anything they could trade.  They could even make a last ditch stand, where they wager their kingdom against the attacking player signing a 50 turn peace treaty.

And then, better still:  Special game mode:  Welcome to the Tournament of Champions

At any time, during tactical battles or out on the world map, players can host a tournament against their rivals.  You can choose not to participate, but then you pay part of the prize money to the eventual winner.  So, in an 8-player game, a 1-on-1 match would earn the highest payout for the winner, since six other players would have chosen not to participate.  Players send one of their champions to the tournament--a special squared circle as a tactical battle map--where they fight to the last man standing.  Losers in the tournament suffer exactly the fate of losers in combat.  But the winner takes all the prize money.