Well I came up with what I thought the FHWA would mandate: something even worse! Like what they did to the old cut-out US Highway shields.
Worst Interstate Highway route marker.
Then I came up with my own idea for rectangular Interstate Highway route markers and a way to accommodate the 238 Freeway in California and the Bud Schuster Freeway in Pennsylvania.
A regular two-digit Interstate route marker.
A regular three-digit Interstate route marker.
This one is for the Bud Schuster Freeway. The number I-99 can be retired, or used east of 95.
This method of numbering an interstate with no available two-digit number in sequence follows the British method for numbering A-routes classified as Motorways.
This one is for the 238 Freeway in California.
And here is something extra: an Interstate Parkway (Jed A. Wilkinson's idea) following the present Shield design..