Mehve, 1 upgrade could give more than 1 attribute.
I just used an example on an item that only has two.
Like say you have the rings.
Ring of Fortune - $2500
Health Regen: +0.42/sec
Attack Speed: +10.5%
Energy Regen: +25%
Ring of the Ancients - $5200
Health Regen: +0.69/sec
Attack Speed: +17.25%
Energy Regen: +40%
All Father's Ring - $16200
Health Regen: +1.8/sec
Attack Speed: +45%
Energy Regen: +100%
You would start with just the first one. One path would upgrade, health and health regen and armor maybe, and maybe the other energy, energy regen, and attack speed.
Do first level of both, and middle path at the bottom would be like the all-father ring. The left and right end path something different.
There does not need to be, and there SHOULD NOT be a bunch of branches, one for each attribute. You'd choose the group of attribute buffs you want the most.
Also, the possible upgrades could be shown when you hover over the item in the shop. I've kept it simple with the few paths.
And you could get +armor from BOTH the breast plate and the boots..
And while you could go both the armor path on boots and breast plate, the bonus at the very end would be different.
So whether you choose the boots or breastplate for armor would be based on the other base stat(like maybe the breast plate gives health and health regen, vs the attack speed of boots.) and the last upgrade you get from following a path.
I original thought that just a simple lvl 2, lvl 3 upgrading of items that'd boost all stats kinda like you mentioned in the end of your post(so ring of fortune would become ring of the ancients, would become all father's ring). But I think this would be much better as 72 items could be made out of 24, instead of 24 better items made out of 24 weaker ones.
When you hover over the shop you could get something simple like.. (photoshopping, brb)