here's a neat idea. back equipment that gives better effects the more rads you have. this idea kinda comes from a game called Face of Mankind. in FOM you could get various mods that ran off bio energy. why not add a couple to fallout 3. you could have:
regeneration mod, heals you .1X how many rads you have(rounded down)
shield mod, increases you DR
cloak mod, give stealth boy like stealth
adrenaline mod, increases your weight capacity, running speed, and melee weapon damage
sight mod(would actuly have to be glasses), increases your chance to hit in vats