Stuff I'd want to avoid:
- The actual Diablo-style loot drops. I wouldn't want guns that had level requirements, but did more damage. I'd rather the damage scale naturally with your own level (if that's a sort of thing the core system does).
- I'd like to avoid a ton of fiddly bookkeeping. Any time I think about how I'd home-brew up a system for this, I immediately run into a ton of "pretend our PnP game is a computer game", which is ass. Maybe I use tokens of different colors to shift around. I don't know.
- Smaller numbers. I'm not worried about the super number scaling you get as you gain levels in BL or Diablo. Ideally, I'd like to be working with single and double-digit numbers for health and damage.
That being said, I have been bouncing ideas around in my head for how I might approach this if I home brew it from scratch. I know some of these ideas have their own problems, but I'll just sort of brain storm here to see what looks salvageable to anyone with more experience in non-D&D games:
- I was thinking about using a flat d20 as the core resolution for attack rolls. Add your bonus(es) to hit, overcome targets AC (or equivalent). Cover bonuses would be important in this system, and many targets would be unarmored.
- I'm not sure if I want a damage roll or just a flat number. Either way, if using D&D/Borderlands-style HP, I was thinking about scaling raw weapon damage by level. Something like an extra +[W] damage every odd level. So, if your pistol does 1d6 damage, and your rifle does 1d10, at 3rd level, they do 2d6 and 2d10, respectively, and three dice at 5th level. I'm not sure how necessary this is.
- I don't want to do much at tracking ability scores or skills, in the D&D sense of the word. I figure anyone who is a Vault Hunter is competent at murder hoboing on an extremely lethal planet. I'll assume relevant checks mostly scale with level. I absolutely don't want some type of "aim" ability score, seeing as how that's the core of the game. Everyone is expected to be a competent shooter.
- The game will feature skill trees like in Borderlands, but with way fewer points and levels. I'm thinking two points per tier instead of five, and way less fiddly math. There won't be tons of +X% to Y. It will work in flat d20 bonuses, or bonus damage dice.
- I'm not sure what I want to do about tracking bullets. I've read threads here on this very subject in the past. Maybe you roll a "reload die" based on the magazine size of the gun. Maybe a natural attack roll of Y or lower (based on mag size) triggers a reload. I don't know.
- I'd like to handle firing from the hip vs down the sights, but I don't want to make the game too complex.
- As far as handling what you can do in a round, I was thinking about doling out Action Points each round. You get, maybe 4 APs to do stuff. You can move or not move, independent of your actions, but moving gives you a penalty to attack rolls. I was thinking about using APs to track fire rate. So, a slow gun might take 2 APs to fire, but a fast gun might shoot multiple shots on a single AP. I don't know if it's worth tracking changing targets with APs when aiming down the sights, or not. It might get too fiddly. I was hoping to capture different feels between running from cover to cover gunning down anyone near by, or methodically taking down threats with a sniper rifle.
- I don't have much experience with initiative systems other than D&D. Any good ideas on what to do?
- I figured I'd use BL's fight-for-your-life mechanic instead of D&D negative HP. I'd probably give you something like three rounds to score a kill, and you can be brought back for a full-round's worth of AP is someone revives you. You lose a round on the FFYL duration each time you get dropped in a single combat.
- Other than that, classes will largely be based off the game, with skill trees fairly close to what's already there (mostly modified to reduce bookkeeping).