I agree with Beezle. A one game friendly takes care of all the race bias you worry about and has more significance than using concedes as a tiebreaker.
Using concedes as a tiebreaker is completely arbitrary. Sometimes, life happens. The loss and giving away your MVP should be penalty enough. Also, it would further encourage "soft concedes".
If a one game playoff isn't viable, I think TD differential is better than concedes. Again, part of the bias is gone since it's two teams of the same race. Although they may have different opponent races, those changes are largely normalized over 42 games.
Here is my wildcard suggestion that sort of implements your concerns of having 4 teams from every race qualify over a 3 season spread. I call it that rather than a pure intra-race competition because that is really the end goal that seems to be sought. That goal can be achieved without discounting as many races as often however.
Here goes nothing:
Take the top 8 wildcards out of the current format for season 1 of the 3 season spread. Those 8 races cannot qualify as wildcard the next 2 seasons. Season 2, take the top 8 wildcards that aren't those races and those races cannot qualify as wildcards the next season. Season 3, take the top 8 wildcards from the remaining 8 races. That way, you get 4 total qualifiers from each race over 3 seasons without completely dictating which races they will be. This structure still creates one season with predetermined wildcards, but it's a compromise that hits your goals and allows a little more inter-race competitiveness which the player base seems to prefer.
Been sitting around since turn 3 in champs ladder because this pixel hugger lost a man. I will avoid the old name and shame for the time being assuming someone is going to contact me for their name.
Why isn't there something in place that concedes them if there is no action over X turns?