Actually, Slovio is not a good example. That project has been dead for almost seven years now, and even before that, it never had more than about one dozen of people who could use it to some degree.
As for Interslavic, perhaps one the long term it would be possible, but at present I don't think the language would be ready for it. And now that I think of this, here comes my fourth requirement:
d) The language should be STABLE.
If a language is in full development, the odds are that at some point the author(s) decide to make improvements in vocabulary, grammar and/or orthography, which would immediately invalidate all previously written material. That's something we can't have. Even the differences between two versions of Novial have been a bit of a problem on the Novial Wikipedia. Volapük is another example of a language that exists in two versions.
These are problems that can and will be fixed with time. But the problem of a small vocabulary cannot. In the case of Klingon, there is only one person who is entitled to coin new words, which happens scarcely. At present, the dictionary contains ca. 3000 words, in other words, not even a fifth of what would be needed for the purposes of an encyclopedia. In the case of Quenya, the situation is even more complicated. AFAIK Tolkien himself left us ca. 2500 words (Sindarin even less). There is a community of people who coin new words, but the non-canonical words are not recognised by everybody. All in all, creating a Wikipedia project for such a language would be asking for trouble.
Cheers,
Jan