I like those basic guidelines and your ideas of product types.
I agree that writing to the "canon" should be the first criteria. Anything added (but especially races, places, significant cultures, powerful individuals, or history) must not contradict the established Shadow World timeline and power structure. And while there are relatively isolated regions of the world (mainly thanks to the Essænce flows), there are few (if any) areas that don't have some interaction with Loremasters, Navigators, and the Unlife. If there's a "great evil" its very likely some local manifestation of the Unlife or one of its servants.
Anyone looking to submit content should be required to read the Shadow World Player Guide (GCP-SW-1001) and it should be used as the primary standard of whether the content fits into the world. At only about 50 pages, it's an excellent but concise overview of what makes the Shadow World unique. Of course, since it's aimed at players, it skips over some important content, so the Master Atlas and Powers of Light and Darkness are also important references for any content creators.
Second in importance to me is that there are already enough (too many?) world-level events, groups, and powerful individuals. What we need is more local and regional content. If someone really wants to write about the world-level events, then they should be fleshing out aspects of the latter parts of the Grand Campaign or giving more detailed adventures set in existing strongholds of the major powers.
One thing that isn't particularly important to me is which Rolemaster rules are used. Shadow World characters have never been entirely bound by the standard rules of any Rolemaster version. Many standard Rolemaster races are out of place on Kulthea, even more so for RMU's new races. And an idea based around Vancian magic ported to the Shadow World is going to be out of place. But generally as long as the concept doesn't directly contradict what can be represented in Rolemaster and doesn't violate the established world canon it should be fine.