I don't think this game is designed well for optional paths in dngs.

The carrot at the end of ANY stick is just cosmetics (Glam, minions, mounts, Hairs, titles) or vertical stat growth. That's it really. Level cap dngs can never have gear that's better that the raid gear cause that raid gear is made as a carrot/ reward for the 2% of folks who actually bother to do the hardest content. Without raiding your best stats come one way or another from grinding tomes which are the reward from completing a dng.

"Have a path that leads to a rare crafting mat" Not great. That is already the goal of maps (the stick), to have rare mats drop that are NEEDED for cool glam (the carrot). If ALL you want is the mat then you will do which ever is easiest, right?
Make it a different mat that's needed? This game already suffers from mat inflation, too many things in this game are only used to make 1-2 items because they need to keep giving crafters reasons to do XYZ content ... People are not gonna be happy sitting in a dng for 45-50mins just for a chance to get a mat they may need more than 1 of just to make a glam. And if you don't make it rare it will become worth less in no time and "Uhhhg, this tank is taking us down this path to get a thing that goes for 8K on the MB?!?!".

If this game had horizontal gear growth then sure, I could see going to a leveling dngs for a hand gear that has a trait or skill that's good for a level cap build you're making. And that special gear could be on an optional path (that you use PF to farm as to not annoy the people who don't care about that optional path). That's how some MMOs work and how I HOPE the next big FF MMO works. But that aint how this game works.