I should be resting for my trip tomorrow. Instead I'm browsing the Elements forums.
Anyway, IMO AI3s should not only be fun but also be good sources of cards for the players, to maybe decrease the time they spend farming AI3 for that card pool. You could point to AI2/1, but due to the semi-random decks of those AI levels, those wouldn't be good sources of cards. And yeah they can get those cards from the Bazaar, but a few more free cards from spins can't hurt anyone.
With that in mind, I've put together a list of cards that current Elder decks lack. I can't predict how useful this would be, or whether or not making new Elders focused on including more of these cards would help new players enough to make a difference. This list is just for reference. (Note: Nymphs are of course not included; the Red and Auburn Nymphs show up as Relics anyway. Also, please point out any mistakes I made.)
4t3 4t4 4t5 4tb 4tc 4vg 4vh 4vi 4vj 4vn 52i 52l 52m 52o 52s 52u 542 55p 55u 562 58p 58q 58s 58t 591 596 5aa 5c1 5c3 5c9 5f1 5f2 5f4 5f5 5f9 5gi 5i5 5i6 5ia 5ic 5if 5ig 5ih 5jm 5l9 5la 5lc 5ld 5lg 5lj 5lm 5od 5of 5oh 5oi 5ok 5om 5ri 5rj 5rm
5rn 5ro 5ru 5um 5ut 5uv 5v1 606 622 63a
Unless I miscounted (a large possibility), there are 70 non-rare cards that can't be won from the AI3.
Some statistics: Death, Earth, and Light have the most unused cards in Elder decks (7 each), while Gravity, Life, and Aether have the least (3, 3, and 2 respectively). Make of that what you will.