I'm pretty sure there has been no announcement. I take your point about derby attendance, but I'm not sure it's worth the complications. First, some teams lack a true rival. Others share them. For example, arguably we, DC and Philly would all want it to be the Red Bulls. It's weird to think how that is true, but it is. If we win that tussle, and DC gets Philly, well...
NYC-RB
DCU-PHI
ATL-ORL
CLB-CIN
TOR-MTL
And then you're left with Chicago and New England. Those natural rivals.
Then you've got the West. Start with Vancouver, Seattle, Portland. How do you handle that?. It seems preposterous not to pair Seattle and Portland, but what, Vancouver/San Jose?
SEA-POR
VAN-SJ
LAG-LAFC
FCD-HOU
RSL-COL
Leaving SKC and Minnesota. Try convincing those 2 teams, or Vancouver and SJ, or Chicago and the Revs, they should give up a game against us, or Atlanta, or LAFC so they can play each other 3 times.