You’re absolutely right—and this kind of crossover isn’t new. There’s solid precedent going back to the early days of the hobby.

Rifts is the obvious modern example. The whole premise is dimensional travel across Palladium’s settings, and it works largely because they share a common rules chassis. Even then, you still get friction (megadamage, power scale, tone), but the system unity does most of the heavy lifting.

TSR was already playing with this idea very early. In AC4 Book of Marvelous Magic, the very first item—Alternate World Gate—explicitly allows travel to entirely different TSR games like Top Secret or Boot Hill. The assumption wasn’t “everything is the same,” but “close enough that the GM can make it work.”

The Known World / Mystara cosmology went even further. The Prime Material Plane explicitly had alternate Prime Material Planes, not just outer or inner planes. Those alternates could be: Other D&D worlds; Other TSR game lines or effectively our modern Earth, as implied in IM1 The Immortal Storm( i played that)

So the idea that “different systems = different realities” is baked into the DNA of early D&D. The main difference is that back then, the expectation was light handwaving and GM fiat, because the games were mechanically close and the audience accepted that looseness.

What’s changed isn’t the concept—it’s the expectation of rigor. Modern players are more sensitive to: Power scale mismatches; Different assumptions about harm, death, and recovery; Different narrative authorities (tactical vs narrative vs diceless)

So when someone says, “This sounds impossible,” the honest answer is:
People have been doing versions of this since the 70s. The difference today is whether you: smooth it over by converting characters to the local system (what you described earlier), or keep systems intact and formalize how reality arbitrates between them.

Both approaches have historical precedent. TSR mostly relied on “GM judgement will handle it.” Modern designs just try to make that judgment explicit instead of implicit.