I'm going to agree fully with Kael Hate here.
Only some of these bonuses would really have any great effect upon matches with the false gods.
He has already made the points and posed the question so I won't repeat them here.
That said, I think this would be a good idea to implement as the game is currently suffering from a lack of diversity fuelled by balance issues.
Icy made the point that the bonuses should be used to further balance the individual elements against each other and he's completely correct in that. I'd regard elemental specific bonuses as one extra factor in detemining the strength of a deck, next to offensive power, stalling power, permanent control and healing power. More factors means more variance in the decks we see and use in PVP.
I would suggest making the bonuses completely unique. Deaths bonus is great - no available card can do something like it and it ties in and is hugely useful to the element. Life and earths bonuses are less spectacular, being available in cards that already exist and serving more inactive functions. Original, exclusive bonuses that the player must actively use would make monodecks more interesting to design and play. (Read "actively use" as the "bonus requires participation in order to work to the fullest" Death, light and time are good active effects. Gravity, life and earth are not. I guess what I'd go for is a powerful permanent spell type effect)
In terms of story, I think it is fantastic that rainbow decks are needed to defeat the false gods. It conjures up the idea of the elements setting aside their differences in order to defeat a common enemy, each elements strength playing a part and countering each others weaknesses. A sort of rebellion of all things against those who sought to dominate them, led by the player who united them. I normally rate the "story" of a game as high as it's gameplay though, so I might be overdoing that a bit..
Anyway, I'm glad you made this thread. Your ideas really would open up the game quiet a bit more, if not necessarily in facing false gods.