OU is a possible long-range option for the Big Ten, although I still think the Grant of Rights for all leagues is a stumbling block. The Big Ten itself doesn't have any desire to challenge the strength of Grant of Rights arrangements since it has one itself.
The GoR is an obstacle to moving from one TV network to another, but not necessarily between conferences on the same network.
Suppose a team moves from ACC to SEC, both ESPN properties. They've assigned their media rights to the ACC, who has assigned them to ESPN. The SEC teams have assigned their media rights to ESPN. So nothing changes! Meanwhile, the ACC is still required to pay the school $20 mn/year or whatever the ACC payout is for the media rights (to SEC games) that they've transferred to ESPN. ESPN doesn't care whether it got the rights from the ACC or the SEC, it will televise the games however it likes. The ex-ACC school has to live with ACC payouts rather than SEC payouts for a while, that's the only real difference; otherwise, they're in the new conference, and as far as fans and players are concerned, it is identical to them being a normal SEC member.
Odds are the conferences would work out some deal in which the media rights and payout responsibilities were transferred to the new conference, just to simplify contracting. The money transferred to settle would be very small.
Even if a team moved across networks, eg ACC to B1G, the GoR wouldn't necessarily totally preclude the move. The content would be diminished in value, because ESPN can't market B1G games as well as Fox/BTN can, so both ESPN/ACC and Fox/B1G would be losers. Yet the ACC would still have to pay the ex-ACC team $20 mn per year. Again, both sides would have incentives to work this out; but the worst case, that they don't, isn't terrible for the school -- they still get an ACC payout and play B1G games.
This is why it was essential for the ACC to keep their exit fee in addition to the GoR. It is also why the ACC school vote for the GoR was unanimous, while the vote for the high exit fee wasn't. The GoR costs the schools nothing if they move, while the exit fee is a major barrier to leaving.
The function of the GoR is really to force a larger re-negotiation when teams move. It brings the networks into the negotiation, and that is why ESPN was willing to sweeten the ACC pot in exchange for the GoR. I don't see it as a huge barrier to re-alignment. Nevertheless until there have been some moves done and deals made, there may be some legal uncertainty around them which slows realignment down.
The B12 is in a special situation, as its media rights are shared by multiple networks. So the contracts must be quite complex and have various exit/termination clauses for different scenarios. I think it's a near certainty that if enough B12 schools want to move, the GoR will be dissolved. It just means that a sizeable number of B12 teams have to find new homes simultaneously. That's possible with Pac, SEC, and B1G all interested.