EzekielRaiden
Follower of the Way
Three possibilities:personally i'd be inclined to just make elemental/genasi the blanket 'G-E neutral' species, otherwise how do you determine which way someone leans when their parentage is from a chaotic good entity or any of the other three corners, are they aasimar or genasi? roll a metaphorical dice or what?
1. Once they're written, use the full "X+Y" options, getting a bit of stuff from both (not sure how that would work, but hey, it's a theoretical option.)
2. Planar essences don't mix. You inherit one or the other. Perhaps in-universe it's random, perhaps not. Player decides is how I'd run it. Think of it like a single gene locus that doesn't intermix. (e.g. you can have yellow beans, or red beans, or green beans, but you can't have yellow-green beans or green-red beans.)
3. Come up with unique effects that happen for things like this. Maybe have a table of random effects, or publish a supplement with the corner and central parts as special variants. (E.g. if a tiefling and an aasimar had a child, they'd be TN planetouched under this method.)
Or, as always, the fourth possibility: don't worry that much about it, as it's a fairly niche concern to begin with.