I don't think it occurs to most people because I don't think most of the community has played many other CCGs like Magic, and doesn't know how things could be. I could be wrong, but Elements is more of a casual gamer's game, it seems to me, anyway, based on most of the forum posts and attitudes I see.
I think the first step is to increase the card base in each element. A lot of elements still don't even have some of the most basic concepts. Life has no midrange attacker, (nothing between cockatrice and dragon) for example, gravity doesn't have a cheap attacker (at least unupped; upped it has graviton guard), light has no unupped midrange attacker (has archangel upped), and so on.
Once the game expands to include some of these basic factors, then we can start worrying about "expanding the horizons" of the game, as you put it. For something like momentum, gravity should have the -best- momentum cards, but giving them to other elements in weaker form could be feasible. Look at things like creature removal; most elements have some form of it. Fire has the most removal, but most other elements have some as well, even if it's not very good. Even Life has Thorn/Spine Carapace and Light has Holy Light/Flash. (though that's very narrow in scope)
Zanz is most likely quite busy with other committments, and doesn't have the time to build the number of cards we need. He'll get around to it in his own good time, but remember, he's giving us this game for free, and he's not getting anything out of it except a few donations and another project to add to his coding portfolio. I'm happy to take whatever he gives us. But yes, if Zanz had infinite time to expand the game, it would definitely benefit from what you're suggesting, at least to a point. Giving Fire a freezing effect would be ridiculous, but giving something like a 2/2 momentum creature to Life, or a 2/2 immortal creature to Light isn't completely out of the question, as two basic examples.
Each element should have some things it's -best- at. No element should be able to do everything, however. Fire's got the best removal, life has the best creature casting cost to power/toughness ratio (but still needs more varied creatures), Life and Light have the best healing, Time has the best card advantage, Aether has shrouded creatures and quintessence, and Light has shrouded and mirror shields. Entropy has most of the cards with random effects, Gravity has momentum and gravity pull, Air has the airborne theme, Death has the death trigger theme, water has the freezing theme, Earth has burrowing, and so on.
Other elements have some of these effects, too, but weaker ones. Darkness has removal, but less than Fire. Fire has some very efficient creatures as well, at least in the power department. (not in toughness, except the nymph) Darkness, Gravity, and Entropy can heal, but not very well. Aether has card advantage with mindgate and fractal, Time has Anubis, Life has Jade Shield, lots of shields have random effects. Fate Egg is also random, every element has at least the dragon that flies, Time and Earth have delay effects similar to freezing, and so on.
