It's all a bit of a blur but I seem to recall information saying that there'd be an entirely new element which'll be required as super-fuel.
Given that there is a big hole in the Raw Mat's table, it seems likely FDev might plug that by introducing one of more new "G5" raw mat's rather than, say, by scooping maguffin-dust from neutron stars or whatever.
Course, the bad news, there, is that if the new mat' is a G5 raw mat' then chances are it'll be just as hard to obtain as other high-tier mat's - otherwise you'd be able to collect heaps of it and trade it for every other raw mat.
Personally, I don't reckon Carriers will interact with planets in any way.
No gliding down to the surface or gathering fuel-mat's from the surface.
It'd be nice if the fuel was available from Bio POIs but that'd cause people to moan that it's easier for Horizons owners to get fuel.
Put that stuff together and it seems likely that the fuel-mat' will be something that's found in rings or asteroid belts.
Hopefully it'll be asteroid belts, just to make it easier to find.
All just a stab in the dark, though.