Maybe I'm not being clear.
So when it comes to subclasses, backgrounds, feats, etc, WotC clearly wants people to expand on those things. So they provide some examples of those things in the SRD.
They did not do this with Bastions, even though it is precisely the kind of thing that WotC would want 3pp companies to embelish (just like subclasses, feats and more).
Do you really think WotC is planning Lord Neverember's Guide To Bastions and is therefore trying to undercut 3PPs?
Looking at their SRD 5.2 FAQ, they say this:
Why is [class, spell, monster, etc.] not in SRD 5.2?
SRD 5.2 includes a wide range of content from the 2024 core rulebooks, but some classes (such as the Artificer), species (like Aasimar), and monsters (including the Beholder) have been excluded. These exclusions are based on brand identity protection, licensing strategy, and intellectual property rights.
Further, in SRD 5.1 there was confusion around whether any named monsters, items, etc. that appeared in descriptions but not as stat blocks were part of the actual content of the SRD, so we’ve taken steps to ensure that there’s no further confusion there. Names like Strahd, Orcus, and Tiamat won’t appear in SRD 5.2.
SRD 5.2 is designed to give creators a strong foundation for building original material, not to replicate every element of the D&D brand or setting. Where content is omitted, creators are encouraged to design and name their own equivalents.
<<<
Thinking about it as a publisher, I’d probably create my own home base system instead of using bastions because it isn’t clear if WOTC considers that their core IP or not. I don’t think they’d send a C&D if I did but it’s not an impossibility.
There’s this big gray area between stuff they clearly think is fine which they’ve put in the SRD and the core IP we know we shouldn’t use like Greyhawk, Mordenkainen, or the Zhentarim. I think bastions fall into this gray area but it doesn’t feel like the sort of thing WOTC would mind people expanding with their own rules as long as they’re not copying the whole system out of the DMG. It also doesn’t feel like a copyright or trademark infringement to do so and it doesn’t feel like your taking money out of WOTCs pockets or exposing material they don’t want exposed that way.
I’d feel comfortable making bastion enhancements for the City of Arches in a pdf I could take down easily if I got hit with a C&D but I probably wouldn’t build a board game around them that would bankrupt me if I hat to take that down.