Now that's actually a good idea. Care to share what type of project you have in mind, or they implemented? We've thought about something similar but we're unsure of exactly what type of forum wide project people will actually particpate in. An art contest with a prize of some sort every so often might work, but as Yop stated, not everyone here is an artist.
I'll try and give a recap of stuff they've done over the hiatus. I'm not a member of the forum so I'm not completely in the know, but I check the site often enough to keep up to date on things.
Also because its a TV series as opposed to a video game series, not everything is going to be applicable.
-They did a complete rewatch of the 5 seasons of the show, and after every episode encouraged a discussion on it. This kind of thing doesn't translate perfectly onto a FF game, but I'm sure it is possible. Have people play through a section of the game at a time and then reflect on their experiences, thoughts on how the story is progressing, whether or not it was a good or poor part of the game. The same way that who would disect and criticise a chapter of a novel or TV episode.
-They've done numerous Top 50/Top 25 competitions on everything from quotes to significant objects, and a significant post to explain it. Whereby the thread starter would post one and then tease the next one in the series, people would try to guess what came next and win points in return. At the end the person with the most points would win. no actual prize involved but like could encourage some investment on their part. This kind of thing could be directly transfered to the FFVII universe easily.
Moreover, If staff was int he mood to pool their money and purchase a prize for the winner of some talent competition. There's no reason to limit it solely to art. Given that TLS has a relatively intelligent community, you could offer competitions in writing the best essay on the FFVII world/themes/characters/etc, or even who can compose the best short story.
All of these things could be advertised on the front page too. So it gives the impression of a constantly active community with lots of projects going on. This probably more than anything else will be more likely to draw in new members. Nobody wants to join a forum with a front page that hasn't been updated in ages.