I think that's essentially still '3' but with a perhaps more metaphorical "you wouldn't look there accidentally" step.4) It's something that no-one (or comparatively no-one) think could be Raxxla, and so remains overlooked. It may have been already 'found', or clues to it have been found, but overlooked because it's an 'outside context' solution to the mystery.
And possibly a "the theory also needs to state why this is a satisfying solution" / "it should be reasonably straightforward, having found it, to convince other people that you have" requirement that a basic "it's 0.1 LY from the system star in this precise direction" wouldn't.
(So e.g. my "it's Ra 10 a" theory is perfectly valid as a solution in terms of "no-one would expect that" but fails on the "no-one believes it either" step)
Certainly my favourite theories - in that they both tend to provide something actionable to check and an explanation for why it's not been found yet - tend to include some element of those....and see if there's anything that people aren't considering because it's outside the context they've been examining.
- What are common assumptions about Raxxla?
- What do people assume at a base level about the solution to the mystery?
- What areas of the game environment aren't being considered as solutions to Raxxla?
- What might Raxxla be if it were something very unexpected (whilst still being viable in-game given the limitations)?
I think a reasonably safe assumption, given the statement that Raxxla is in the Milky Way, is that it does have in some way a defined location, and it's not such an abstract concept that it couldn't have a defined location. Beyond that I think it gets very tricky to say anything for certain. I quite like the idea that the defined location is a system which does not appear on the galaxy map, and must therefore be reached by doing "something", for example.