Alliances should be, at least optional, not based on a single gameworld.
There should be an extra part for alliance management, including a member base administration with different flags (for example: standard member can join every game, maybe member has to apply, blocked member due to bad experiences) and an extra presentation page that is accessable from everywhere, so people without an alliance and/or gameworld yet as well as visitors could get themselves a picture of the alliances. Those alliances could have a subdomain, too, like http://XYZ.alliances.airwaysim.com
or whatever and either be in a plugin (like the current alliance thing) or free in some kind of HTML or whatever coding.
Additionally in the member administration there could be a public part where members can apply for an airport (or give three to five wishes) to plan for new gameworlds.
To boost general forum usage all systems I can think of allow, basically, cheating. For example achievements could be tied to the forum and/or some kind of like/dislike system for single posts.
Also a "forums newsletter" could be sent via the ingame PM system, presenting interesting discussions, things people can decide (like this thread here) or just a reminder there are forums.
I made some good experiences with such newsletter on an alliance level here in AirwaySim, pointing out important threads in the alliance forums etc.
At the end I just want, without any judgement, say what I found to be the most interesting forums to participate:
Forums where the developer(s) are active. Not only short worded, but informative, maybe even small talking now and then about background information. I always felt I have to attend the forum to gather this information and I liked when I knew my ideas would not only be read, but also noticed and maybe even implemented.
I'm aware of the fact there is a bug report and a feature request forum, but sometimes information there is rare as well as no general information. To give you an example: I just asked around why the new gameworld doesn't run until 2030 (what would make sense because you would have to introduce A350 and 787) but not even the people active for the last year could give me an answer, they are sure it wasn't even ever mentioned.
And that's how humans work: At the places where you can get something (and let it be "just" information) they gather and participate. That's why a market or a church are more crowded than that street with normal houses.