Oh hells yes. This idea was made popular in Sliders. It is one of my favorite ideas about parallel realities. One ep of Sliders featured an evil double of the inventor of sliding of the opposite gender, so keep this in mind.
Sliders was one of my favorite shows; although, I tended to watch it irregularly. I don't think I saw that episode but I do know about it.
The otherworlds in this campaign will include several of my homebrewed worlds, some new worlds, and the core D&D worlds such as Greyhawk and the Forgotten Realms. I'll likely use Beyond Countless Doorways with it as well. I haven't decided yet.
Slider-like Earths might be possible but I don't want to focus on that (*). More specifically, I want to create a connection from Kulan, Maran, and Rielun to AlterEarth. Regardless of which campaigns that AlterEarth connects, this campaign will be a d20 Modern campaign. Thus, if characters were to travel to AlterEarth from, say, Toril, those characters would have to be revised into a d20 Modern format.
Portals may have existed in AlterEarth's past (likely) but either these portals went dormant or were closed by some unseen force. Now, the portals are opening again.
The "current year" of this campaign will be, at earliest, 2020. That's when things start to become more "adventure worthy" as borders between certain otherworlds begin to invade upon AlterEarth.
Rielun is one such world that will "mix" a lot with AlterEarth but it is primarily one-way towards Rielun. Kulan's link is more restrictive as I want to keep it isolated from too much advanced technology. As a result, Maran is even one step further removed from AlterEarth since its link is only through Kulan.
I haven't considered my other "otherworlds" yet.
Connections are stronger to Oerth and Toril and weaker to Krynn and Mystara. Athas doesn't have any connections but The Great Wheel does connect to AlterEarth through the D&D Worlds. (The connection isn't direct.) AlterEarth isn't directly connected to my Mirrored Cosmology but that cosmology is accessible, tentitively, through Kulan.
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*Gamma World, likely. Dark Matter, not as likely. Guardian Earth, um, no.