The Chronicles of the Elkandu:

The Elkandu Universe was originally devised as a crossover hub centered around the Nexus of Torn Elkandu, which allows travel to different universes. In addition to that, however, it has developed into a bizarre mutant monstrosity of its own spanning twenty years of writing, millions of words of stories, several MUDs, a decade of roleplaying, multiple crossovers, and way too many characters.

Karzan Universe - Dancing on the Edge of Death:

An alternate universe fusion of Freelancer, Warhammer 40,000, and maybe some other stuff, plus furries for some reason. Starts with "Earth Conquest", or skip that and go straight to "Dancing on the Edge of Death", doesn't really matter. Itself gets crossed over with Elkandu and other stuff starting with "Way Past the Edge of Death and Into Oblivion", which leads into the Interdimensional Bridge series.

The Interdimensional Bridge series:

Crosses over with both Elkandu and Karzan, as well as a number of other settings. Prior reading isn't necessary, however. Starts with "Dancing Out from Oblivion".

Stormseeker Saga:

Connected to the Elkandu, but mostly takes place in other universes. The best place to start is either "Balance of the Force" or "Cycle of Nightmares". "Balance of the Force" takes place chronologically later, but was written earliest and is arguably a better starting point, while Cycle of Nightmares is chronologically the first in the series. "Legacy of the Shapers" and "Blight of Dragons" should be able to stand on their own.