[William]

I will not spoil the fun by joining in the enumerated questions and answers, but the existence of something like this in the GURPS system was precisely my purpose behind writing up my "Tomos, the Divided Land" setting for crossplanar GURPS fantasy. It was article number 6203 in Pyramid #2, December 2006.

The basics of crossworld travel in the setting are as follows. Writers in this thread are welcome to take them and run with them.

1.) Posit that any naturally maniferous world has natural gates to the elemental planes, particularly at least one stable, human-transitable gate to the Plane (or Decan, etc.) of Elemental Earth. The key is that such a plane is likely to be understood and known to almost every magical world.

2.) Posit that this Plane can be traveled between portals in a reasonable time and level of safety, that it is susceptible to the creation of permanent gates with the Create Gate spell, and that gates here can be found to and from new worlds (either unknown but associated to some token item or person, or simply seeking new gates to explore) with various divinations. (They may be very hard to find, or unusably one-way, such as the portal to Yrth.)

Voila, a system for mages to travel from one fantasy world to another.

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The "home culture" of the setting was Tomos, a Greek-style democratic civilization featuring orichalcum and alchemy, on a world full of island archipelagos with no major continents. The central hub for crossplanar travel was Katakoptron, the Mirror City, sitting in the glassy border elemental region between Earth and Air (and hence featuring solid ground and breathable air). The City is the usual central site for the creation of double-wide permanent gates between worlds formally allied in the trade system, putting trade routes more convenient to one another than the haphazard placement of the natural gates. Worlds that could be reached included a few mentioned in Banestorm, namely Loren'dil, Gabrook, and Olokun, with the full complement of GURPS fantasy creatures, along with a few others of my own devising. The elemental planes could also feature an array of interesting, useful, and competition-inspiring resources.

I have long thought about proposing to SJGames that I expand this setting into a longer pdf. I have plenty of material. I know, however, that "my homebrew setting" is not an item high on their wish list.