I find that leveling up shields isn't really worth the crystals invested, a lot of the time. Unlike armor and weaponry, the only benefit of heating shields is a slight increase in defense (which already isn't as useful as status resistances). To encourage a more even distribution of fire crystal usage, I think we could give a larger benefit for investing time into heating our shields as well. I have a couple suggestions for what these bonuses could be:

1) Decrease shield recharge delay - Basically, lessening the time it takes for the shield to start recovering, given that no damage is taken.
2) Increase shield recharge rate - Pretty self explanatory.
3) Increase the amount of defense given per level - Just to make it more notable

If any of these were implemented, it'd just be important to make sure nothing is made TOO powerful, if that makes any sense. :P

I think that, with armors, the +1 health boost also doesn't scale very well with the increase in base health as the stars of equipment go up. In Tier 1, going from 5 pips to 7 is a great bonus, but going from 13 pips to 15 in Tier 3 hardly means anything, especially with the increased enemy damage. I'd love to see the bonus become a little better, but I don't know how much would be a good value to have it still be balanced.