I think an easy way to work it out would be to go with the highest you want in Attack and Defence.
If Max End Game gear has Attack of say 2000 and we think that the current level cap is 40, than at 40 items are all in the 1950 to 2000 range, 39 - has a range of 1900 - 1950, 38 has 1850 - 1900 and so on always with just a little overlap so at level 1 we start with something like 5 - 50. I don’t think any weapon would have a 0 Attack.
Defence could be worked out at say 50% of Attack so you will always do damage.
Before anyone jumps all over this and says that’s always half my hit points or health, that’s where affixes and leganary effects come in to mixup the difference, some tanky builds will only take 20% per hit, glass cannon builds my have a chance to be one shot or 75% in one hit.
And by end game I think this is where Sets start so they should be at this Level Cap, no more than 10-20% or each of the next tier of items then as we pick up our first top end legendary items the range can go up the same 10-20% so as to keep the over all numbers as low as possible.
This is far from a perfect option but more of an idea that will be easier to work with and with proper handling with keep power creep at bay.
Yes this is what is done in D2 I’m not a fanboy but it looks like the best option at least make it so the best combinations work with white items, and keeping another item type viable.
If that means white items have 3-4 sockets and other items only have 2 all the better.
You can still make the basic Runes work as they are now with 1 of each, but in a white item it could open up more possible combinations, without the stat bonus will it be something better for your build?
Wow I think that person did a thesis on Diablo, way to much for me to take in, but a very hand thing to show the Devs.