ISPs already do that on Settlement Free Peering (SFP) links. I've yet to come across an SFP agreement where they didn't have provisions about link upgrades when capacity is reached by BOTH sides.
What they don't have is provisions that say that if one side saturates an SFP link, the receiving side is obligated to upgrade to support that because that undermines the whole purpose of SFP.
If I have an SFP agreement with overage provision and I send twice as much traffic towards you as you towards me, I pay for the overage similar to a straight paid peering arrangement.
Paid peering congestion is purely contractual. If I pay for a 10gb peer connection with you and traffic from my network to yours saturates it, I pay for a higher bandwidth connection or accept the congestion.