I don't really do a lot of tweaking as such, I symthesize instead. I have become very adept at taking a bit of material from here, adding some more from there, making a few minor rewrites for consistancy, and BAM...something different on the whole from it's original source material.
My current Homebrew is a prime example of this. I could not find a post apocalyptic technomagic game that I wanted to run, so I started with the perpetrated press's Arsenal and Factory, added in straight 3rd ed D&D (that quickly became 3.5), a dash of Dragonstar (most of which has since been replaced with better ideas and material...I think only the schools, a few spells, and a couple PrCs survived the final cut), and got a deal on ebay when I was trying for a copy of OA and ended up with OA, Rokugan Campaign HB, Magic of Rokugan, Samurai book, Ninja book, and Creatures of Rokugan (all for $65), so Taint (which I already wanted to add in like the chaos from Warhammer) got added in. Since I love the artwork and miniatures of WH40K Eldar (and much of the fluff material), I made them into the new elves, and the shugenja and samurai from OA and Rokugan were perfect for them. Add in IK mechanika rules, spice with various d20 Modern materials and PDFs from RPGnow, a bake in my brain for too many years. All while trying to actually run the game.
So far my players seem to find the recipe quite tasty, but it may still end in disaster, as I think I am beginning to drown in material. It is frustrating, finding that perfect rule, spell, whatever, but it needs to be modified to be consistant with this other rule, spell, whatever that you got from a different source, which naturally changes other things.
ARGH!
Anyway, I could stop at any time. I do not have a problem, and will be finished modifying the rules as soon as I can get my hands on Races of the Wild.....oh wait, maybe I should also add in dragon material from complete dragonkin....but then Sandstorm is coming out....aaaaaaahhhhhhhh Help me, I am drowning in a stream of consciousness!!!!!
Hi, My Name Is skippy, and I am a RPG Material Tweaker...