Keeping things within the general feel of a series' setting while creating your own spin on things - how to do this?

I was reading the SoYouWantTo.Write A Fan Fic page and I've decided that in my attempts at writing Fan Fic, I'm going with either a Divergence or Adaptation fanfic (or possibly Alternate Universe).

I may introduce new characters not seen in the original work's canon.

Last time I asked about The Stations of the Canon, and decided to take the advice suggested and not be afraid to change canon in response to what happens in your fic, as was suggested.

My problem, is how far to go with Broad Strokes and stay true to characters' personalities and gimmicks.

I've been writing a Ben 10 fan fic that's more Alternate Universe Fic - these indicate it's an Alternate Universe Fic a bit more:

• First of all, it's still Ben Tennyson, but an Alternate Self that's a little bit different: he's not as arrogant or cocky as in canon, and more of a Nice Guy who's got Adaptational Intelligence and a Functional Addict to fast food/sugary food.
• Alien X doesn't have Serena and Bellicus, and is a bit more klutzy and How Do I Shot Web?, and some changes to the Celestialsapien species' origin. I'm also trying to make Alien X less of a Story-Breaker Power than he is in canon. How to make
• Ben likes to use Shocksquatch when he can, as compared to the canon version of Ben.
• I'm trying to work out how to make Atomix less of a Story-Breaker Power (he's kept for Rule of Cool reasons) yet still workable.

I'm trying to make this a Lighter and Softer plotline, but how far with Broad Strokes have I gone?

This is basically a Lighter and Softer fic that's more about the action and aliens he uses than anything like Shipping or anything emotional.

I've got the basic plotline in place, and am trying to develop a one-shot antagonist (but avoiding him becoming a Generic Doomsday Villain is another problem) for Ben to fight against.

I've been Google-ing to try and find scripts for Ben 10, but not sure where's the best place.

Basically, I want to retain the general feel of Ben 10 's setting, yet make this a complete Alternate Continuity in Broad Strokes.

If anyone could help with how to make this interesting and avoid the issues mentioned above, I'd welcome it.