To encourage user activity perhaps add in a modification which would allow users to see the most recently replied to threads (typically lists 5-10 threads) at the bottom of the forum, also you can add in things such as top reputations, most posts, etc. Also perhaps a shoutbox? Although a shoutbox limits user activity at times, it also allows for more of a community feel, or if that becomes an issue for vip+ if need be I'll help find the modifications.