If I’m understanding your question correctly, you are talking about giving each pony a sort of ‘sub-element’ in addition to their original?
That’s actually kind of interesting so here’s my idea.
(Sorry, no RD as magic. In fact, I don’t think any of the others really fit with magic so I’ll just say that Twilight gets ‘double magic.’)
First up: Applejack.
Somewhat reflecting that old argument that AJ and RD’s elements were swapped during production, I’d say that AJ would be an honesty/loyalty hybrid.
She even calls herself ‘the loyalest and most dependable of ponies’ in the show (Applebuck Season, I believe.).
A little tougher but I’d give Rarity a dash of honesty to go with her generosity.
It’s not a perfect fit as she does stretch the truth at times, but she is certainly willing and able to be very straightforward and honest when it comes to her artistic and aesthetic vision.
In addition to her kindness, I think generosity is a decent fit for Fluttershy.
Despite her fears, she is typically (if somewhat reluctantly) willing to help her friends when she can.
(Actually, this is largely given by process of elimination. She’s definitely not honesty, loyalty, or laughter so there’s really only one option left.)
On to: Pinkie Pie.
I don’t think that it’s too outlandish to give Pinkie a sub-element of kindness.
Being the element of laughter and her desire to bring smile wherever she goes plays a role in this, too.
Now, she does have a habit of being annoying which may seem ‘unkind,’ but this is never done intentionally and always comes from a deep-seated desire to make others feel better.
And finally: Rainbow Dash.
Not too tough to see Dash as a combination of loyalty and laughter.
Her jokes are a little more barb-filled, but they are typically all in good fun.
Besides Pinkie, Dash probably laughs more often than any of the other primary characters and, while her emotions are rather volatile, she does tend to be pretty jovial and positive most of the time.
AJ = Ho + Lo
R = Ge + Ho
F = Ki + Ge
Pinkie = La + Ki
RD = Lo + La
So that’s the way I would split things up in your hypothetical situation.
The traits seem reasonably plausible to me and the elements are nicely permuted through the rest of the group.
Wow, I actually typed all of that out.
I think it’s time for bed now.
4 a.m. is too late for log diatribes on odd pony topics.