Don't get me wrong, screwing around and making your own fun with other players in a spontaneous way without any real goal or measure is all very well by me, I enjoy having no real goals set for me in freeroam and just interacting with other players.
But I feel it could also do with a little spice, and this would all be completely optional. When challenging another player to a deathmatch, I think it would be much more interesting if you could set a bet limit. This would be limited by your cash I suppose. If the other player doesn't have enough cash then the amount played for is simply what he has on him. Winner takes all. There would be no restrictions at all, you could use any weapon, you could get one kill and run like hell, its all in the air. Going into these are completely up to you. I think this would make freeroam way more interesting for lower level players who actually have skill and want to take on higher level players.
Likewise, higher level players would have intense deathmatches if they decide to play with millions of dollars at stake.
You thought ego battles in freeroam were intense, this would make those shootouts with other cocky players even more intense.