Many melee bots use convenience targeting (aim at the closest opponent). Because of that, the dominant strategy for over a decade has been to stay away from everyone to avoid being targeted. If no one is shooting at you, you effectively have 100% dodging and perfect survival. And survival score counts a lot in melee (each bot you outlast means extra 50 points).
But in teamrumble, then it is a completely different story. There are 2 aggressive movement teams in the top 5. If there is a single melee bot ramming against you, shoot power 3 bullets and it is toast. If there are 3 team bots ramming against you, you're toast.
I presume you, as a game theorist, know about the Prisoner's dilemma (probably more than I do). Do you think melee bots will ever be smart enough to cooperate and form temporary teams? Or would communication be too difficult?
I'm initially skeptical it would be advantageous. Against the weaker bots, you'd only be able to eek out a small amount of extra score, while against the stronger bots you're temporarily teaming up with, you have a lot of room to gain points by killing them instead. (E.g., going from 50% to 55% is a lot easier than going from 94.9% to 99.9%.)
But upon further thought, if two top bots already know they'll get about 50% against each other whether they team up or not, why not save that battle until the end and maximize the score vs everyone else by killing them all first. I don't really see it ever happening, but maybe it could be viable.
Edit: Using the terms from the Wikipedia article, I'm trying to say that I'm not sure R > P. You may have relatively little to gain by maximizing score vs the weak bots, and you have much to gain by improving score vs the other strong bot. For the weaker bot, of course it's advantageous to team up, but it may never be for the stronger bot. (I'm also presuming that any bots trying this would be the strongest bots, which I think is likely.) (And of course this is all complicated by differing performance in Melee vs 1v1, too - teaming up with Diamond and then facing him 1v1 every round might be a bad idea.)
I know about prisoners dilemma. I also know about trigger strategies and sustained collusion.
Meleerumble is not a prisoner's dilemma. It is a constant-sum game (all bots scores always sums 100%), while prisoner's dilemma is not. In constant-sum games, collusion is never sustained.
But 2 player sub-games do have prisoner's dilemma aspects. If 2 bots collude, they both increase their chances of survival against the other 8. If 2 bots shoot against each other, they both diminish their chances of survival. If one shoots but the other not, the one shooting gains an advantage (earning points and gaining energy).
Don't expect existing bots to learn collusion as most bots have no active developers looking after them. The rumble is full of hardcore defectors and hardcore cooperators, and in prisoner's dilemma, the best response against hardcore players who pick the same strategy regardless of what others are doing is to defect against them as well.
For collusion to work, even temporary, at least 2 bots need to go for it. If you upload a single trigger strategy bot, it won't work. If you upload a bunch of trigger strategy bots, like 300, then it might work.