Thoughts on format selection procedure for the regular season:

First of all, here is my opinion that drives the rest of this: Format bans are inappropriate for best of 1 matches. This leads to situations like we saw in Season 4, where deathline never played Row Control, presumably because it was banned by their opponent most of the time, even when deathline was the lower ranked player.

So, I want to come up with a better formula for format selection for best of 1s. The mathematician in me says that this means deciding on what should be the desired outcome at the extremes of ELO difference, and what should be the interpolation between the two extremes. We could also decide what we even mean by "difference" here, but I would insist that this is already defined by the ELO system to mean arithmetic distance.

So, here's what I personally would try:

In the extreme where player A has infinitely more ELO than player B, I think player B should simply choose the format to be played.

In the extreme where player A and player B have the same ELO, I think both players should choose a format, and they should be chosen between with 50% chance for each. (This has problems, but I have more to say later when we consider the season as a whole to hopefully address them).

So how do we interpolate between these two extremes? One choice is to say that the lower rated player always chooses the format, and the 50/50 only happens when they are equally rated. I actually think this is a pretty good option if we do implement what I think for the season as a whole. We could also do this, but with a larger threshold range for when the 50/50 happens, such as when the ELO ratings predict that neither player has a greater than 60% chance of victory.

Now, in a vacuum, the above rules are kind of bad, and strongly favor the selection of the lower ranked player, even when they have similar ratings. This is why I propose we implement Dave's Stupid Rule: You can't choose to play a variant against somebody that you have already beaten them in this season. So, if player B has won in invasion against player A, B cannot choose to play invasion again against player A during the same regular season. Note that A is still allowed to pick invasion against B. Also note that this rule applies whether or not B was the one to choose invasion in the first place. This rule will prevent a player from spamming a format that they prefer, but they can still reap the benefits of their preference at least once. And I do think that a sufficiently lower-rated player should get to reap those benefits, at least once. (and if somehow, you've already won in all variants, but are still the lower rated player, then I guess you get to override this rule but this would never actually happen lol)

I think that this combination of rules could work quite well for the top of the standings, where we expect many rematches. It's a bit more unclear for how it would play out in the middle-to-bottom of the standings.