I suggest you increase the cost but allow the shield to also count Life quanta, since it is of its element. Also, let both versions have Life, Light, and Water. Just make the upgraded version a bit more cheaper. You could argue that Earth should be another one of the elements, but I can do just the same for Fire, Air, Death, and Time. Maybe even for the other elements if given enough time.
This isn't heavily over powered as it doesn't reduce damage. Reducing damage is better for a lot of small hitters (Skull Shield vs a lot of Malignant Cells), while healing is better for a heavy hitter (Abomination vs Shard of Gratitude; overall 0 net change =D). Like most cards, this is situational.
Anyways, other than what I've previously said, great idea!! ^^ Just find a better picture and I'd be totally fine with whatever you decide, as long as you keep the whole healing thing going =P. Also, I like World Tree, but maybe Tree of Life? xD Cheesy, no? ^^
.... These are some new ideas if you want to change how the healing goes. You could make it the reversal of Fahrenheit and make it heal x/5 (doesn't have to be 5 but it's an example =P) where x is the number of
you have, and make it have an upkeep of a few
quanta. Or you can make it so the elements build upon each other. If you have Life quanta it heals for 3, if you have Life + Light it heals for 9 and if you have all 3 it heals for 27 (ie the general formula is 3^n, where n is the number of the 3 elements you have). Encouraging Trios!! =D Unfortunately it also buffs up Rainbow =P