There's already a guide on the Combi system posted, if that's what someone who clicks on this is looking for go to Heiliger's Combi Guide in the Tips & Tricks section.
It's no secret that players have self combied, and I don't know if there's a way to prevent it. I think the way one receives Combi Exp (I'll call it CXP) needs to be tweeked. Right now the only way is to stalk the pre-channel select lobby in hopes to find rookie ranks (if you wanna do it legit). Any one of them can decline you if they wish, but more on the CXP now.
1: Increase the CXP obtained from a Rookie you're combied with leveling up (maybe from 1 to 5)?
2: Make it so that playing a match with a player you're combied with, and that you're also teaming with rewards you with 1 CXP.
This would hopefully make the players feel like they don't have to self combi because they could just get CXP by playing with their friends. Sure they could farm it this way, but I think having them creating accounts just for it is a bit more annoying.
I always thought the Combi Master should give you 1 of every item, as of right now it gives you everything except a Pet, Weapon or a Skill. There're many ranks beyond the current final prizes tier (Combi 36) that serve no purpose.
C-Lvl 41: Black Dragon (Statless)
C-Lvl 46: Black Katana (+1%)
C-Lvl 50: Dual Mastery (It kind of fits in with the Master Vibe, this skill represents Balance).
Also I don't think it'd hurt to add collection set bonus for the Master Set with an appropriate name tag that gives off that 2008 S4 vibe. It'd also be another way to show you have max combi since you'd need to have all of the parts in your inventory.
Master Set Collection Bonus
-EXp+20% (Was going to put "Katana Charge-time reduced" here, but then that'd give some form of advantage).