I think that depends. First off, how would you have the 5E version interact with armor. Personally I thought a combination of Armor as Damage Reduction and Defense Bonus By Level would have been fun to try in 3E, but never did.
I think that I would try running it as half your proficiency bonus as a prototype. This bonus would just be added on top of your normal ac. Some example AC follows.
[TABLE="width: 700"][TR][TD]Armor[/TD][TD]Normal AC[/TD][TD]Variant level 1[/TD][TD]Variant level 20[/TD][/TR][TR][TD]Plate, Shield, Defensive[/TD][TD]21[/TD][TD]22[/TD][TD]24[/TD][/TR][TR][TD]Leather[/TD][TD]17[/TD][TD]18[/TD][TD]20[/TD][/TR][TR][TD]Monk level 1[/TD][TD]16[/TD][TD]17[/TD][TD]N/A[/TD][/TR][TR][TD]Monk level 20[/TD][TD]20[/TD][TD]N/A[/TD][TD]23[/TD][/TR][/TABLE]That would let your defensive abilities grow with experience without making it easy to get maxed out, impossible to hit AC, and without requiring an extensive rewrite.