@Garanwyn, this was fascinating to me, as - while unsurprised that the SGG team is aware of the high win rates for raids - I was unaware that this conversation was occurring. Honestly, I do think the win rate in the upper echelon of the game is too high. At the same time, I actually think it’s probably too low in earlier phases of the game. At the least, I think that raids are too frustrating until you near the endgame (decision to NOT link to multiple threads demonstrating this, all of which I’m sure you’ve already seen). Weirdly, I have already written about this dichotomy in this exact thread.
Anyway, at the risk of having actual content in this thread… 
Move the Exponent
I believe I’ve seen Kerridoc suggest that simply tweaking the location of the exponent in the (assuming this is even correct) damage calculation would mitigate the power of color stacking. Has that suggestion been discussed with the SGG team / does it have any traction?
Mitigate the Stack
My recent experiences with the five star raid tournaments that disallow an element has really driven home to me how hard it is to win without color stacking. I have often struggled to defeat teams with a five star (often emblemed) tank that I can’t bring any of the strong element against, presumably because the raid tournaments are rigged I lack a roster replete with maxed heroes that would allow me to stack off color / mono effectively.
In other words, in terms of those five star tournaments, I’m at a point analogous to what I wrote about earlier. At some point earlier in the game I could raid with my best rainbow team, or stack, though the cost was a subpar team power. Now, in these tournaments, I can stack less to bring a more powerful (on paper, at least) team, or move towards mono, by bringing in weaker and weaker heroes.
I realize I’m making my point in a rather tortuous fashion here, but the point is…find a way to disincentivize the player from stacking (or from stacking the strong element). The raid tournaments do this with an artificial constraint.
The Best Solution
Live PvP raids solve this. If I’m attacking a team while it is simultaneously attacking mine, both players (mostly) have to go rainbow. If I go blue, my opponent will obviously go green, gaining an insurmountable advantage (funnily, even moreso than a normal raid now, since they’ll be stacked strong while I’m stacked weak).
It would be incredibly exciting to draft and place your heroes, taking turns with the opponent. The strategic ramifications of each choice and the live aspect would be a new and (I think) enthralling game component.
As an additional benefit, since raid teams would mostly be rainbow, people would be completely unable to complain about the boards being rigged. And, since raid wins would be zero sum, another thing people would be entirely unable to complain about would be any artificial methods of choosing winners, setting win rates, cup ranges, etc. In fact, I guarantee that there would be no more complaining of any sort.