Much as the subject is saying. 
Also change an Arcane, get 10 random Runic traitstones or choose 7 runic traitstones.
Also change a Major traitstone, get 5 random Minor stones or choose 3.
Other thoughts?
would be very nice if it would be implemented something like this. 
Here’s how I’d imagine an exchange rate working.
Exchange for less: 1 Higher Traitstone for 1 lesser Traitstone. So exchanging a Runic for a Major would yield 1 Major.
Exchange for equal: 2-3 Equal Traitstones for 1 of another color. So exchanging 3 Runics would yield you 1 Runic of a different color.
Exchange for more: 5-10 Lesser Traitstone for 1 higher Traitstone. So exchanging 5 Green Runics & 5 Yellow Runics for 1 Arcane Light Traitstone.
From an economic standpoint this is the best way to incorporate such a system without offering to many resources to the player. Even if someone disagrees, this is the most likely outcome should an exchange ever come into fruition.
ew.
5:1 ratio, both directions
5 minors > 1 major = 1 major > 5 minors, and so forth.
as for runics to arcanes, can always do 3 of each color,
My conversion rate would be the following:
Upgrading
Minor 3 = Major
Major 3 = Runic
Runic 2 and 2 = Arcane
Arcane 6 = Celestial
Downgrading
Celestial = 4 Random Arcane
Arcane = 3 Runic
Runic = 2 Major
Major = 2 Minor
That is much harder than what you suggested, by more than half a reduction.
I doesnt agree with the runic to arcane and arcane to celestial, nobody would had given 6 arcane for one celestial, I do believe. Also I think nobody would downgrade a runic for 2 major stones. Also getting 6 Arcanes from a Celestial would be too much, considering getting an arcane is just 1% harder than getting a Celestial.
Tell that to my arcane stealth stockpile.
Hey, it’s more generous than my prediction. Then again mine isn’t random.
Regardless, my guess is something like an exchange system is already in the planning stages, and the numbers will not be “equal” or “fair” by any means. In which case, beggars can’t be choosers.
While I like the idea of being able to trade traitstones, I think the game has been set-up such that players pretty much have to hang around for at least 20 weeks (for weekly bundles) in order to acquire all the arcane stones you need - though probably longer, as it can be tough to accrue >50 arcanes in any week using this method. It’s a good way for player retention.
Having said that, the issue is redundant resources as @Tard_Carnival indicates. While there is a chance that a Spring Imp will pop-up to require another 16 arcane stealth, it is likely that many people will accrue resources that they can do nothing with.