Hello reader! Currently I am making a Runescape (popular mmorpg) D&D. This incorporates and replaces the skills of the normal D&D rules and replaces them with Runescape ones. The only thing that I have not changed is the D&D style combat, however I have run into an issue. To those familiar with Runescape, there are many armors which you can buy or smith or gain from monster drops that you can upgrade your gear with. I want to apply that here as well. As listed below you will see the stats that each piece of armor has. 

The issue that comes from this is that certain players will quickly reach high armor stats whilst others might not as quickly using for example the magic combat style. This would make make players with melee invincible as most npc's will not hit on something with 50 armor. To balance this out i could make the npc's also very strong but that would make players with low armor fell proportionately disadvantaged. How do I make a tanky player not invincible while still being fair to a less tanky player?

This can also be seen another way around: How can i down scale the armor per level whilst still making them desirable enough to obtain?

One idea that i had come up with is that depending on the armor difference, a player would throw different dice. For example: A player that has 5 times the armor of its opponent will be able to throw a d20 vs the opponents d4. This makes the opponent still have a chance as he can roll a 3 on his D4 if the player rolls a 2 on his D20. Players wuold have to throw a defensive roll.

Since armor in d&d are in sets already, you can just make the AC bonus of each runescape set equal to the average of each piece rather than the total. If AC gets too high too quickly, you can turn a few points of AC into a point of damage reduction.

Also remember a d&d character's AC doesn't change much through a campaign. It might go from 13 to 18 or 18 to 26 as extream examples.

You can of course buff monsters (especially humanoids) using your house rule armor system, but remember 5e monsters favor HP (or damage resistance) over AC.

Thank you for your input! I will think it over and yes, a lot will have to do with how the Npc stats will turn out as well. For additional feedback, i have come up with this idea, what do you think:

Essentially, you will roll for both attack and defense. The dice you roll with will depend on the difference in armor. I like this idea because this way everyone will always have a chance even if you have the D4 - D20 scenario.

Example: Player has 20 AC and the Npc 10 Ac. This gives the player a 10 ac advantage allowing him to roll with a d20 to a d12 of the npc for both taking damage and dealing damage. The numbers themselves might have to be tweaked a bit and play tested.