I was trying to subtly raise the possibility you are putting way too much faith in "body slots" as a solution. I wasn't keen on entering that part of the debate while the basic "does 5E stop working without magic items?" debate was still raging.
With that mostly settled, I can confess I don't see 5E has nearly the kind of stacking problem that motivates adding a formal body slot rules framework.
And TBH I didn't think body slots really solved any balance issues in 3E either. They were good minmaxing fun because they added texture; something to wrap your minmaxing around. But any gamer worth his salt weren't stopped by hyperspecific nonsense like "you can't benefit from both a medallion and a brooch at the same time" anyway.
(Yes, I had to look that up)
But I feel that level of detail is inappropriate for 5E.
Insofar as three attunement slots feels overly crude and restrictive in 5E, I would encourage you and everyone to look for other solutions. With special emphasis on things that
actually solve specific problems, rather than just decorates them (like detailed rules about how many rings you could wear would).
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Maybe take a page out of PF2 and go like this:
- you now have 10 attunement slots
- every existing major item now costs 3 attunement slots
So far you've basically blown up supply and demand three times (with a 1/3rd added for good measure).
But now you have design space to designate less awesome items to use only 2 or 1 attunement slots.
This way one character attunes to three major items while another attunes to ten individually less impressive items. This way you would attempt to mitigate a real problem with 5E; the fact that so very many cool and evocative items go by the wayside simply because you already have three majorly impactful items, and you can't attune to any more.
But maybe, just maybe, you'd be willing to give one of these up, if you could get the cool and evocative item PLUS something other as well?
(The comparison with PF2 basically ends at having 10 attunement points. In PF2 the rule is: you can only invest 10 items during any given day. 5E items are allowed much more variety in power, which is why the comparison ends here)