I'm not confident that SA:MP has the luxury to implement an AFK system since the playerbase is low, even AFKers are essential in boosting up someone's motivation to visit the server. I mean, 1/200 is always better than 0/200! As for that, since you can detect when someone is AFK maybe you can count for how long to and subtract it? I thought about that but it might eat too much of the server's resources to execute.
Instead promote businesses and make a top 3 of the most selling+visits/2 businesses. Most 2 active groups counting time online from when half the /groupmembers is together online. Idk, maybe instead of promoting simple stuff promote specific stuff like these. Tbh I like the business idea I just suggested.
Overall, good idea but we need to adjust to the situation, that is a dependent on trying individuals and inactive server, it would make sense for me with a 30-50+ standard playerbase.