Under the Hood/Behind the Scenes Question for Staff: What determines which classes are & aren't supported?

The title itself is pretty straight to the point but to get into more detail on my question: I myself have realized that Blood Hunter uses the Ranger Chasis and seems to be built from that and Illrigger seems to borrow the Paladin Chasis. I was wondering if it is just that you all are able to modify an existing chasis or something else. Because from what I have noticed when you aren't modifying an existing chasis, it appears that there are some issues like for example with the Lord of the Rings classes? 

TLDR; Do you all just determine what can be added based on if you can modify an existing chasis or not?

Remember that they have teams of people working on each individual project.

So team one could put out one class type and the next team can put out the other class type with NO interaction between them. One team uses one standard and the next uses their own that's different.

They more than likely don't have any rules they could tell you other than they try to keep classes balanced. Balanced to whats already published.

If they had rules and set guidelines they would have published them long ago.

If we're talking in terms of which third party classes get added and which don't I doubt there's anything as formal as rules that determine it. I suspect it's more of a case of them trying to implement everything and then realising if it can be done or not based on the limitations of DDB's spaghetti code