The criteria are all about the state of fandom. Is a language (and it’s community) fan friendly, has some competent users, is the language actually complete enough to be usable by an enthusiastic fan, is it active *right now*
Designed to be easy above all, it actually is easy to learn. After about 30 hours of study you know all the grammar and vocabulary you need to know, and switch to the hard task of learning to be expressive. The community is large by conlang standards, small as compared to an urban book club, and has been active for ten years.
This community is so strong you can almost start applying natural language measures to it, for example the number of couples who both speak Klingon (at least one) and the number of native speakers (at least one).
New, but fan friendly. The best are writing entire blogs in it. I worry about if it will have long term staying power as Avatar is probably good for one sequel, but will the language survive a collapse in Avatar fandom, heaven forbid?
Not easy to learn, but has dedicated fans who will do their best to teach you for free, essentially. This is the most alien reference grammar I’ve ever attempted to read, and I’ve subsequently heard that comment from other people. This language has the most promise for making you think different.
The internal logic of auxlangs make this language sometimes a little too friendly. Not only does it want to come over for dinner, but this language would like to stay forever. In any case, this is the only conlang that is reputed to lead to conlang enthusiasts getting girlfriends (and boyfriends).
Very promising. It has all the elements of a fan friendly language. A talented conlang author, a successful TV series and book series. Too soon to see if it will take off, but I’m optimistic.
neo-Sindarin and neo-Quenya
Not fan friendly on several counts. The canon can’t be used to reliably resolve enough to use it as a fan. No conventions. Correct me if I’m wrong, no fans have written books entirely in either, although there are attempts to translate the bible. If neo-S or neo-Q are successful, it is despite all these fan-unfriendly characteristics. Despite all this, there are active forums and people who are competent at neo-Sindarin and neo-Quenya to the extent that it is possible. Maybe the community will rally and decide on what is canonical, either by planned or natural means. I’m counting this as one because it is one community, one creator, and in fictional and real terms, they are related languages.
Stuff that shouldn’t be on my list: incomplete languages, languages you can’t use, languages that are hostile to fan use (like if they are wrapped in copyright protections), languages that have no buzz and no on promoting them, languages that are English con-dialects.
I can think of a few promising conlangs that lack fans. There are lots of promising conlangs that lack someone to finish them. Y’all go out and adopt a conlang so I can fill in the next blanks.