If we can't get some new bleeding edge state of the art mining tech for memory hard calculations then the quick and dirty solution is to reset the N factor back to 1 and have a big relaunch and invite all the GPU miners back in.

Additionally you may decide to reset PoS payments to 1% a year but make them increase with N. e.g. when N factor gets 5 PoS is 5% again N = 6 then PoS = 6% etc

Hopefully by the time we get back to our current N factor (which would take a couple of years) then the technology to mine at the higher N factors will exist.

My third suggestion is that while we upgrade the security of our PoS tech we also change the payout intervals from three montly to weekly.

Will GPU miners mine a coin with 60 million already in existence?

They will if it is profitable.

The difficulty will probably have to be increased.