"The Modern Technique of the Pistol" has a chapter on Handgun Modifications.
If I can excerpt & paraphrase from this a little -
ESSENTIALS -
1. Snag-proof, combat sights (front sight ramped, rear sight devoid of sharp corners.)
2. Manageable trigger (around 4 lbs, little creep, 1/8" or less reset.)
3. Dehorning (no part of the pistol should chafe, gouge or rip one's person or clothing.)
USEFUL -
1. Extended thumb safety.
2. Lower ejection port slightly to improve clearance for ejecting case.
3. Beveled magazine well aids in inserting single-stack magazines. (Not an add-on funnel, which is unnecessarily large.)
4. Throat chamber opening and polish feed ramp for reliable feeding, especially of hollow point ammunition.
5. Round and polish the bottom edge of the extractor, enhances reliability by easing the extractor's engagement of the case rim, reducing retardation of the slide's forward movement and lessening damage to case rim (of interest to reloaders.)
QUESTIONABLE -
1. Ducktail grip safety unnecessary if pistol is dehorned properly.
2. Ambidextrous safeties. (If left-handed, have the off-side platform of an ambidextrous safety removed.) These just invite accidental safety release by clothing.
3. Recoil buffers.
OBJECTIONABLE -
1. Sight ribs.
2. Extended slide stops.
3. Muzzle brakes, including ported barrels and slides.
4. Extended magazine release buttons.
5. "Action tightening" to match standards. Most tightly-fitted pistols suffer reliability to achieve this level of accuracy (there are exceptions.)
The author is Gregory Morrison, edited by Jeff Cooper. This is an excellent book, IMO. Most of it covers technique, but there is some discussion of equipment. Heavily slanted toward 1911 pistol, with revolvers acknowledged.