I get that idea, but making cards with devotion cost more based on the effect you're getting defeats the point.
Traditionally effects that scaled like this (
Molimo, for Eg) couldn't reasonably go beyond what one would call reasonable numbers without some sort of shenanigans like infinite mana combos.
Look at
Wandering Stream. When they designed it, they knew reasonably that it was never gaining you more than 10 life. If things they couldn't predict happened, they were okay if it somehow gained you 12 or 14, since that would present some challanges for the player (presumably).
Bountiful Harvest varies a bit more, but the number of lands a player has over the course of a game is usually less than 7, and doesn't vary but that
that much, maybe 4-5 at most. 12 life for 5 mana might be a bit above the curve, but it's not breaking anything.
Devotion doesn't have the problem. Due to the wild fluctuations in colored mana symbols in permanents (The difference between
Phantasmal Bear and
Invoke Prejudice is staggering with this mechanic in mind), a reasonable prediction of a random player's devotion count in a constructed format varies WILDLY more than other previous countables (like lands).
The ammount of effort for
Wandering Stream to double it's intended effect is...currently not possible. The ammount of effort for
Bountiful Harvest to double it's intended effect is reasonable, but hard. The ammount of effort required to get
Nylea's Disciple to gain double the life people expect from say,
a random 4 drop is not only hard to calculate, but as we've seen from standard from the last season, incredibly easy.
This is why I'm not so hot on devotion as a mechanic. None of the cards with devotion (with the possible exception of the gods) really excite me. Even if everything goes right,
Fanatic of Mogis isn't enough better or more interesting than something like
Hellrider for me to want to add it.
Reverent Hunter doesn't add anything that something like a static 4/4 for
would, in my eyes.
Master of Waves might be the exception, since he's basically a fucking green or white card, but that's not something I want to do right now because making blue more like white or green doesn't interest me.