I know this might be alot of work, but what if rewards for leveling up mcmmo was added. Like with fishing, you get a chance to reel in custom items, may it be armor or tools. Things with fun fish-ish names. Same for woodcutting and excavation. For farming, you could add custom hoes with fun farming names. Adding custom reward drops to mcmmo would for sure keep people interested and online more, less would get bored imo.
I think this is an interesting idea, although I don't really see it being possible. MCMMO plugin is built specifically to do what it does. As far as I'm aware, there's no way of changing lucky drops; let alone being able to incorperate a skill level chance feature. Sorry if that is a let down to you, but like I said, I'm pretty sure its impossible. The only version of this idea working that I can think of, is reward screens that come up upon reaching certain power level milestones, as power level directly relates to MCMMO and can be configured to come up in Custom player screens. My vote right now, with that being said is Nah. I think it would be a painstaking amount of work for what could really turn out to be underwhelming reward.