Currently the Big 10 has 14 football members divided into two 7 team divisions.
Two days ago, the Pac 12 commissioner commented on re-examining conference expansion.
Big 10 membership is far more lucrative than is Pac 12 membership.
Big 10 wants member schools to be in the AAU (academic research related).
The Pac 12 has at least seven (7) AAU member schools ( Univ.of Washington,Univ.of Oregon, Colorado, Stanford, UCLA, UCal-Berkeley, & USC). The University of Utah and the University of Arizona are also AAU members.
Any travel or distance concerns could be minimized by the Big 10 adding a third 7 team division of schools in the far west.
A 21 school football conference would likely lead to more intra-conference playoff games to determine the conference champion & to help rank schools within the conference for playoff bowl purposes.
More intra-conference games means fewer non-conference games which is not good for Notre Dame. Any non-conference opponent will most likely be a warm-up against a non-threatening team,not a game against a potential national powerhouse team.
Because the Pac 12 actually has 9 AAU member schools, a Pac 12 & Big 10 merger of AAU member schools plus former AAU member Nebraska could result in three 8 team divisions or four 6 team divisions.
P.S. TV/media rights contracts for both the Big 10 and the Pac 12 are soon expiring. A coast-to-coast conference consisting of a substantial number of highly ranked football teams could demand record shattering compensation.
Again, good for the Big 10, Pac 12, SEC,and bad for any remaining independents.
FWIW Many high level Big 10 executives / officials have solid connections to Notre Dame which could, presumably, facilitate exploratory discussions. The money is almost certainly going to exceed prior contracts by a substantial amount. (Thanks in part to the increase viewing /watching habits developed due to quarantining due to Covid-19.)