Why is time/aether a rivalry? I see no connection between the two other than that they have a good synergy. They aren't the opposite of eachother.
This has been discussed on this same thread already.
Time and Aether are both "Other Dimension" type of elements who might be fighting for control of that dimension or something.
Not all rivalries has to be like hot vs. cold, black vs. white, etc.
Entropy/Gravity... I see no connection. Why would they be rivals? (Even though I did put them on opposing teams, it could have just as easily been entropy on good too.)
This has also been discussed.
The way I see it, Gravity represents order, science and things making sense. Entropy on the other hand represent chaos and randomness. If you think about it like that, they are the opposites.