Limitations of the engine... a purely speculative topic, for the most part. Modders might have something to say about it
Just to be clear: what's a limitation for modders (because of poor tools available, hardcoded stuff they have no direct access to, etc, etc) is not necessarily also a limitation for the developers with full access to the source code etc.
There may be some partial overalap, but let's stress it again: "modders can''t do it" about anything is NOT an immediate justification for developers, too.