I sometimes wonder why the 3rd party publishers are bothering with additional player material. I bought the Scarred Lands players guide a while ago because I loved the setting back in 3rd edition. I bought it as a GM but it's chock full of players options, almost none of which I would be inclined to encourage players to use (A barbarian with 1/3 druid casting, a Feat for Spiked Chain users - really?) and it really just highlights that there's not really all that much space really for player material.
Where there would be space is if you were able to tie all the options you gave to specific setting elements or organisations. But that doesn't work too well and if it does, it limits broad appeal anyway - (which supposedly is the reason why setting books are chocked full of player stuff).
There's also the fact that official D&D material is easily referenced online whether through D&D Beyond all one of the many other probably non-legal wikis that spring up. This means that I don't actually have to haul my copy of Xanathar's guide along to the game with me just in case I need to check something. Similarly spells - not only is it easy to look them up online you can also get those handy spell card packs which make everything a lot easier. Once you start mixing and matching 3pp material this becomes a lot harder (and really there is NOT a lack of spells in the game alreay).
I think 3pp subclasses and feats need to be setting specific and intensely flavourful. A lot of publishers just seem to look for gaps, many of which exist for a reason, and then fill them. The most useful material for them to publish is probably GM material - there's always space for magic items and monsters.