The sure fire way to screw up your league and also ensure that no one but a bunch of lugnuts join is to go crazy with scoring stats. 15x15 stats is a joke. Stats like double plays turned, outfield assists, hitting for the cycle etc... they are all nonsense.
5x5 is definitely the starting point - I've added hits & losses to make my league 6x6; while replacing .avg with .obp.
I've got nothing against .avg - but it can be misleading as the end all statistic, hits/abs - instead of using a more plate presence approach, which includes walks (intentional or not) - hits+walks/abs, now we're talking. Walks seem to be overlooked a lot.
Also added hits; even though I was shooting for something like doubles or extra base hits - you give credit to guys who hit homers, steal bases, and hit with .avg - we'll see what happens with this, contact is key & I've already taken away .avg
You have to be even with batting & pitching, so adding losses was the fairest way - some other pitching categories can be a variation of one that your using (ie: why use k/bb or k/9 if your already using k) tried it last year and loved it - but gonna change it to losses this year. You've got wins & losses - how more realistic can you get.
Try some things out, ask league owners what they think & come up with some fun stuff.