By "re-certify", do you just mean the pilots? Or do you include the plane itself?
The original problem was that the FAA requirement for how the stick force has to vary with angle of attack could not be met, because of the new engines having to be so far forward: instead of the stick force continuing to increase with higher AoA, it started to decrease at some point because of the pitch up moment due to the engines.
The simplest way to fix that would have been to increase the ground clearance so the engines could be moved back to where they were on previous 737s. That, of course, would have had to be done in the design stage, before planes started being built. I'm not sure if it would be possible to retrofit existing 737 MAX planes to do this.
If the physical airframe stays the same, then there has to be something in the flight controls that compensates for the pitch up moment of the engines at higher AoA in order to meet the stick force requirement. MCAS was an attempt to do that while keeping everything "close enough" to previous 737s. If that constraint is dropped, there might be a way to keep within the FAA stick force requirement without having the same failure modes as the MAX does, but that would still require a new type certification for the plane.
