I don't really see how this Q is different from Suggested System/Mechanics/Setting Choices for X-Files/Fringe Inspired Game? and What RPG worlds/systems work best together in crossover combinations?
(see the comment by @Pat_Ludwig: "Questions should be specific enough that there can be a single or best answer. (...)")
Obviously, this is not sys-rec, but what is an adventure if not a sub/spec-rpg, and why would this Q have answers that are more objective? Help me see the principles more clearly. Thx. -.-
What's important to understand is that the goal of StackExchange is to help people solve problems. And by people we don't mean just the person who asked the question; we mean everyone who runs across the problem in a future search.
So the first key difference between the questions you've cited is that only two of them have problems to be solved:
Mxyzplk needs an adventure for his group.
Mad Brew needs a system to run his specific campaign.
You need... What? What would solve your problem? A comprehensive list of systems that work well in a crossover? See below...
Next, there's the issue of format. SE is designed so that you see the problem, and then immediately see the solution that worked for the person with that problem (accepted answer) or, failing that, the answer most voted up by the community.
If you're interested, or need more details, you can read further answers. But it isn't necessary to read every answer ever posted if you're just trying to get your problem solved.
Eventually, Mxyzplk should accept the adventure that worked for his group.
Eventually, Mad Brew should accept the system that he used for his campaign.
What would you accept as answer? What about the answer you accepted solves your problem?
Finally, there's the issue of scope. StackExchange works best with narrowly scoped questions that are quite specific. Constraints breed quality. This is true of creative writing and troubleshooting.
Mxyzplk needs a level 6-8 Azlanti themed adventure for Pathfinder, that fits in with the campaign scenario he described.
Mad Brew lays out seven bullet points worth of constraints.
All you ask is that systems be "good for a cross over." This is rather vague.
The way I describe it, is that "good questions have at least one, and fewer than infinite answers." If anything within the set of things being discussed meets the criteria for the question, that's bad. Robert Cartaino and Jeff Atwood explain it in Good Subjective, Bad Subjective and Real Questions have Answers.
Addendum: Good Azlanti Dungeon Adventure? v. Suggested System/Mechanics/Setting Choices for X-Files/Fringe Inspired Game?
The difference here is in the answers. Both questions are on-topic, reasonable questions. Both questions become at-risk of closure if they attract swarms of "list all the things" kinds of answers.
That said, the X-Files question has a greater risk because it's criteria is inherently looser. Because RPGs can be re-skinned and repurposed, and are generally more versatile than adventures, there's a higher chance that people will start suggesting random things without stopping to think how well they know the answer is true.
Addendum 2: What RPG worlds/systems work best together in crossover combinations?
Here's my take on the closed question:
You ask for things that have actually been run, which is very good.
The definition of what you're looking for seems to be too vague to me.
How I would improve it:
What kind of cross overs are you looking for? Peanut-butter-and-chocolate synergy, or zany dissonance? Time travel? X vs. Y in a fight to the death? Etc.
How do you want to go about crossing over? Similar systems? Different settings for the same system? Extremely different systems that you can weld together into an unholy monstrosity?
You mention some example crossovers. What worked about the examples, and what didn't?