How bad is a level 20 fighter failing typical Will saves?
Are they pretty much auto-fail?
Are the DCs at end game so high that they would still be auto-fail if you buffed the Will saves?
I was thinking about trying to shore up the Will save weakness with a green steel necklace:
+6 Wisdom (+10 diplomacy and haggle)
blindness ward and disease immunity
+1 Will save (exceptional)
That would give me +4 to my Will save. Eventually, when I'm swimming in scales and upgrade to tier 3 I would add +2 Will save (exceptional) for a total of +6.
I think this could be worthwhile if the +4 bonus is regularly going to come into play. But if I'm still auto-failing Will saves with the +4 then I'd be wasting an item slot.