Here are the ideas of my brother concerning Role playing rules and especially mixing up and making full use of all editions of Dungeons & Dragons.
Again I’ve been thinking about remit for Dungeons & Dragons #5e. You need a combat mechanic that lets you use previous versions with little modifications and is quick and easy. I am throwing this in for comment:
Roll the attack against AC, touch or a fixed save like ref, fort or will+10. Damage is equal to the points you score over the target UP to the maximum of your weapon plus Strength bonus plus item bonuses. If an ally boosts your chance to hit, you chance for getting higher damage is automatically raised but still not above your maximum so no power creep. Every game since D&D 2nd Edition has had attack listed as ‘claw +3, 1D8+5’ so that is the monsters skill; +3 vs AC with a max damage of 13. GM’s can do that conversion in their head. No equipment needs changing (and can still be listed as dice so that the 5e books be used by people running 3e or 4e games).
Spells roll to hit against their normal save plus 10 and maximum damage is whatever maximum the spell could score, so a fireball from a low level mage would likely do less damage than one from a high level mage because the master mage is going to score higher on average. Auto hit spells like magic missile are still rolled on the usual damage dice (d4s). Sneak attack damage is the same; damage scored with your weapon plus a random d6 or two.
If you roll a natural twenty, damage is till calculated as normal but attacker gets to roll on a critical table. Crit tables are a different topic but I would have a table where you roll a d20 plus your level and item bonus of sword, wand or whatever. Certain magic items can give you the ability to reduce crits scored against you so that players have some defense against crippling crits but few monsters would except the BBGs.
I would make the attack skills similar to the current classifications, based on players buying feats, not skill points. Warrior feat makes weapon skills available to be used untrained but no bonuses. Mage feat makes spell skills available, Cleric feat makes divine skills available, Rogue, etc, etc. Trained feat gives you a plus to weapons or spells of that group (melee, simple ranged, healing domain, necromancy etc) of, say, +5. Focused gives you bonus to weapon or spell types (swords, spells with fire descriptor, etc) of perhaps +3. Specialised gives you a bonus to a particular weapon (short swords, fireballs). This feat tree makes it possible for one player to have skills in all groups but he is not going to be as good as a character who specializes in one or two areas.
The feat trees can be attached to classes if you still want classes or you can have a class-less game and players just buy into the feat trees that they want. I’d still keep leveling though, otherwise it isn’t going to feel like D&D but more like one of the ‘other’ games out there! Lol.
Of course wizards can’t buy skills in spells above their level yet so no first level casters casting fireballs, etc.
Starting characters CAN use weapons that they aren’t trained in but untrained penalty (-4?) will discourage that. A first level fighter COULD have an exotic weapon (say a repeating cross-bow) but he would need Warrior feat and Trained exotic (repeating crossbow) before his skill is any better than -4. He could take a regular crossbow and his Warrior feat would put him at +0, taking Trained simple ranged, would then put him at +5.
Wizards can cast any spells that they like and know but I would probable use either a spell point or a Vancian systems to prevent the wizard just plastering an area with fireballs until he is the only uncharred corpse on the battlefield. I quite like spell points (characters get points equal to the total levels of spells that they can cast per day) but from personal GMing experience I have known that to be abused. I allowed the wizard to regain 1 level per hour of activity so that the party didn’t have to keep stopping to let the wizard rest, with all levels being regained after 4 hours of meditation or 8 hours of sleep. Clerics, of course, have their god gift them with knowledge of all the suitable spells of their level so everything works as in previous editions too