Agreed on active suspension: it's too complex to introduce mid-season but the rules for suspension could easily be loosened. If the cars appear dangerous they will be forced to act.
Again even loosening rules on suspension would
be complex and unworkable in the short term.
The fact all this porpoiseing occurs at high speed, and there is no way to replicate that in wind tunnels under current rules, means any testing will have to be on track.
The simplest and easiest route for the FIA to go if they deem it dangerous, and want a fast simple easy to implement solution, is re introduction of a plank under the cars.
That forces everyone to run considerably higher ride heights, thus eliminating porpoiseing.
However, it will severely reduce the ground effect portion of downforce, so make the cars considerably slower, but would also most likely make overtaking under braking easier.