[Chris is me]

The idea that the ideal alliance is anything but 3 robots that could do absolutely everything in the game is silly - nothing would have the strategic flexibility of that. Speaking of ideals in the alliance selection metagame is uninteresting and not nearly as fun as talking about the roles alliance partners can and will take.

I think that a tube relay is going to be nearly essential to an alliance at least at the regional level. Almost every robot will be able to hold tubes, but probably only one or two robots on an alliance will be able to reliably score them. That third bot would be best served in being a "tube passer", running a relay from the feeder lane to the caution line and dropping tubes for a robot to score on the floor. They may score a few of their own, especially in the end game, but an alliance with one or two good scorers will want to keep those good scorers scoring rather than wasting their time with retrieval.