Is it possible to set some kind of special MMR gains/decreases for those very weird matches? The reason I say this, is you should want the matchmaking system to be a design that lets you reach Champion 1 in any form, rather than just searching 4s. If you want it to be this way then the follow examples need to be tweaked. Examples:
I understand that Halo 5 might not have the "population" to add the very best version of party restrictions in, so why not just mess with the MMR gains and decreases to better help/nerf the design of the problem areas to promote a more balanced ranked environment. Thanks and I hope you consider my thoughts.
- Players search together on purpose but are way different ranks. They're most likely doing this because the stronger players will carry the weaker ones to a higher rank. (Can the system see this match as an outlier and instead read that group of players as all equal to the rank of the highest rank in the lobby? This way searching ranked encourages similar skill matches every time and if someone wants to go in with weaker players that wont work either).
- Uneven parties match up vs each other. In this example, the team that has the bigger group of players shouldn't get a lot of MMR and the team with the smaller group of players shouldn't lose that much MMR. And then the weird team of three with one random vs two teams of two match up, this is hard to tell what is and isn't fair so it should probably be seen with small gains and decreases as well. Anything hard to tell should be too.
EDIT: Here's a clip of Pro Player Ace complaining about Solo Q matchmaking. In this clip he says he needs a good teammate and asks if anyone will play with him after he just gets done losing a SoloQ game. It would appear even the pros dislike searching alone due to how unbalanced it is.
We already give different CSR in those situations.
Likewise, when uneven parties match up, we also already don't change the MMR much, unless the smaller team wins.
We've also tested using the best player in a party for matchmaking. This resulted in worse matchmaking overall, including less people playing. The problem is that the weird case you're talking about is the exception and not the rule. A lot of people do just want to play with their noob friends. But, to balance this, a very high ranked player won't get much CSR when they do that.
We are not seeing a large imbalance in the data when players SoloQ. If a Pro player queues alone, the system will almost always put all the worst players on his team within the match in order to make sure it's still a 50/50 split. That's balanced, and not imbalanced. Pros should still lose half their matches if it's possible to give them a hard enough challenge.
As to your comment that we should let solo players get to the top of the Ranks, I don't personally agree with that. I think you have to roll with a party of Champs if you want to be #1, just like if you want to win a tournament.