The Leitan Peninsula
Relative stability in the miasma.
The Leitan Peninsula is a, nay the, rock in the turbulent miasma of the multiverse. No matter the timeline, regardless the apparent plane of existence, the peninsula exists. It does evolve, but it exists. The peninsula is a world of magic, technology, and life.
Though listed as AD&D, the game is based in a home-brewed ruleset I affectionately refer to as AD&D 1.5e. It incorporates elements from 1e, 2e, and a few other systems that I've enjoyed over the years. Most of the systems were released in the 1970s or 1980s. There may be a few features from later systems, but this is a system that penalizes players who focus on min-maxing the ruleset and rewards players that focus on storytelling and excitement. Very free form.