If you allowed for full damage would keeping the -4 attack penalty be enough of a balance? I'm kind of "dumb" when it comes to mechanics, so I have to ask these sorts of questions.
Personally, I don't see the need to give it a drawback at all. The feat already gives you a pretty significant drawback no matter what element you pick.
Which is really odd, come to think of it. I think there are very few feats that give drawbacks, and the ones that do only really give a drawback in the sense of "You are now an outsider/fey, which might have negative effects (but also gives positive effects of its own)"
If I'm going to spend a Level 1 Only feat slot AND take a -2 to all will saves permanently, I'm going to want a little bit more than "electrical resistance 5" and "do half damage with electricity on unarmed attacks."
Considering most use cases of unarmed damage are going to be pretty low damage per hit, that's generally going to translate to a single digit, low double digit of damage with the alternate damage type, which means it's vanishingly unlikely that halved damage is going to actually have a chance of triggering the alternate damage type effects, which ultimately just means you can color-code your attacks if they are weak against the static thing you picked back at level 1, and otherwise isn't much useful.
I feel like Elemental Heritage could really use a rewrite in particular, especially considering it uses the DRX/Type notation that isn't really found anywhere in the book, the benefits and costs of each element are vastly different (Gain Bleed and 2 Panache(!!) in exchange for a weakness against one of the most rare damage types, compared to 5 Resistance to another rare damage type, Night Blindness, and the ability to deal half damage with your attack to transform it into a nondamaging attack that also is entirely a save based on damage.
Some of the options in Elemental Heritage are useful, but I think it's the closest thing to a Trap Feat Fantasycraft has, because a lot of the choices just aren't really worth the investment. Well, actually, there is Bushwhack Basics, but that's less a trap feat, and more a feat tax.
They can only take the feat at level 1, just let them get something actually useful out of it.