I will consider what you've said about colour direction in the limited environment. I like the idea of making playing extra colours available but very much rewarding you for having negotiated a main colour in your pile through clever hybrid drafting.
Sounds perfect.
Well when I say onboard crap I mean things that make spells easier to dump (Affinity, echo, alternate costs etc) or things that are really only relevant in play and are kinda non-technical like exalted or extort style abilities right?
Affinity to Red (This Spell costs 1 less for red mana symbol on permanents you control)
Probably t0o Broken. Could do it as alternate cost, like a suspend like version of the gods. Pay X to exile from hand, cast without paying mana cost when your devotion to red is X. Probably not much advantage over the Affinity to a color mechanic. Especially because It adds even more openness.
The Library is a possiblility, but like you said, scry has been done to death lately. Other Possibilities:
Hand - Things that have been done before: Forecast, Splice, Discard Spells, Force of Will, and other "you may reveal" cards.
Graveyard - Done to death, but a favorite of mine.
Exile - To0 many interactions can be confusing.
Stack - Not enough design space.
Sideboard (lol) - Done before with Wishes, etc.