First of all, great concept, Purity Riot. Fun and fitting with the flavor of the game. This sort of thing could be what we need to ever-so-slightly encourage mono decks enough to make them feasible, as a player cannot have two different "planes" helping his/her deck. I really like the feel of each benefit befitting the sorts of things that a deck of this type could do normally, i.e. Air has an Air effect, Fire has a Fire effect, etc. Please forgive me if the format of this entry is obnoxious. Now then...
Aether: Neat, not apparently overpowered. Boosts Aether's two cheapest creatures. I don't know that I would put one in an Aether deck, or that the AI would use it, but I see no reason not to give Phase Spiders and Sparks a bit of an edge.
Air: Okay, pretty intriguing...but as was mentioned above, could be too exploitable. Perhaps instead of affecting power, there could be a small random chance of a Thunderstorm being cast on your opponent's field? Like a Thunderstorm brewing in the area. If you would disagree with Air's randomness, then consider Fog Shield.
Darkness: I can't argue with the justification of Darkness being dark, even if I think this might be overpowered. Perhaps a random chance of a spell "missing"? Is that possible to code?
Death: I can easily imagine this going very poorly. But, once again, the flavor of the idea is rather sound. I have no suggestions at the moment.
Earth: All right. This runs afoul of my biggest objection to currently suggested planar effects, and that is the notion that the effect seems to strengthen/benefit creatures of foreign planes more than of its home territory. Burrow, especially the ability to control it, is a rather desirable effect. I don't know what I would do as an alternative, perhaps make creatures unable to unburrow? Hmm...
Entropy: I agree with ftb above, that casting Chaos Seed on non-Entroy newcomers would be nicer. Concept is fitting in theme and style, overall I forsee no problems.
Fire: Ooh, neat idea. If you dropped the cost-increase, I feel this would be an excellent and balanced addition to the metagame. Something does need to be done to dissuade creature ability-spam. Fire is an appropriate choice for this.
Life: Once again, not trying to make something of your card ideas that is counter to what you intended, but I could see this as being not especially beneficial for the element that controls the plane. What about something like "each Life creature heals 1 damage at the end of turn", or something along those lines?
Light: No suggestion currently.
Time:
I'm not sure, I liked my original up'd idea of non-time cards coming into play stasis'd, but that's too much like other cards. What if it let players with the mark draw two cards per turn?
Actually, I think I like that either of those ideas more than the current suggestion. As stated before, my impression is that these planes should give a slight edge to the element represented by the plane/a detriment to creatures of other elemental types. Your current suggestion for Time is actually not advantageous for time at all, as Time creatures are one of the few not feared for their abilities. If I was playing a mono-Time deck, I would not want this plane to be on the field because it would benefit my opponent more than myself. Unless, of course, the card only allowed Time creatures to be activated the turn they came out. I doubt anyone would be furious about that, might even be fun.
Water: Well, I could see this ability backfiring. If I played a poison deck, for instance, I doubt I would want my creatures stalled when playing against Fire. I don't know what I would do to change this ability, though. Perhaps non-Water creatures get frozen?
Anyway, great job, very creative, and I look forward to seeing more!
