Mainly, I wouldn't. Because if I'm going to the trouble to do something like that, I'm not going to be constrained by some of the 5E oddities, such as the oddball skill list. There is no space on the Venn diagram between "compatible enough to be a 5E clone" and "changes enough to make it worth my time.”
As I explained previously in the other thread, this is basically where I’m at; there’s enough wonky and/or against my preferences in 5e to not be worth me doing a 5e retroclone, but instead do my own system.
My start point was 4E, but with a mission to “question everything” which did result in some changes in the direction of some 5e mechanics, but ultimately resulted in a system as distinct from D&D as Palladium’s system (which is why I felt comfortable with dropping the WotC OGL/d20SRD so I can have my own Open System License with zero ties to WotC).
Like Steven Mitchell, to keep on topic with the thread, here are some of my changes from 5e;
- Replaced ability scores (i.e. the 3-18/8-20) with the modifiers directly (i.e. your score is 3 instead of 16 which generates a +3 modifier). Almost nothing in 4E/5e directly references the score (in 4E only carry capacity and starting hit points reference the scores, everything else uses the modifiers).
- Tweaked the attribute list for non-OGL compliance; STRength, ENDurance, REFlexes, WITs, INTellect, and PREsence.
- Went even tighter on what 5e calls “bounded accuracy” (it increases by about +3 over 15 levels) for combat so that progression is entirely linear along the “hit point/damage dealt” axis. This means ANYTHING in sufficient numbers can be a threat to PCs regardless of their levels (even a max level hero will overwhelmed in a straight fight if outnumbered 20-1 by mooks).
- Loosened up the bounded accuracy for skills and set a number of special maneuver that would be feats and such in other systems as simply higher difficulty actions. Special abilities then give bonuses to those actions to allow those higher difficulties to be reached more reliably. The idea here is to remove the “you need X to even attempt Y” pit that a lot of 3-5e feats created.
- Also tightened up and rebalanced the skill list; Acrobatics (REF), Arcana (INT), Culture (INT), Deceit (PRE), Engineering (INT), Fitness (END), Insight (WIT), Intimidate (PRE), Medicine (WIT), Nature (WIT), Persuade (PRE) and Stealth (REF). Strength doesn’t have any skills of its own because it provides raw ability (lifting capacity, climbing and swimming speed, jump distance, plus melee weapon attacks) that is then pushed with Fitness/Endurance. Several skill uses got moved around; ex. there is no “Thievery” skill because all the picking locks/disabling traps is Engineering and all the pick pockets/sleight of hand is Stealth.
- Four defenses/rolls that use the best of two attributes as their base; Armor (STR or REF + armor), Dodge (REF or WIT), Fortitude (STR or END) and Willpower (INT or PRE).
- Constrained my equivalent of hit points across the board. It’s higher at level 1 fof PC’s (25-30 points) but the top end at level 15 isn’t even 4x that (95-114 points). By contrast non-combatants and weaker critters might have as few as 4 points.
- In the default rules the players roll most things while the GM sets target numbers. Players roll to attack an opponent’s defense target number, and roll their defense against an opponent’s attack target number. There’s a bunch of optional rules for changing who rolls what so the GM can set the system to match his own preferences (ex. Old School would be everyone rolls weapon attacks vs. defense target number and everyone rolls defense vs. spell attack target numbers).
- In keeping with more modern settings and the idea that it’s easier to cull than create, I include a wide array of races; technically its 10, but several of them; beastmen, eldritch and mutants; are basically “build your own” races (with half a dozen or so examples) where a number of different races share a common origin in the default setting.
- I broke classes up into combat-related abilities (still called classes) and non-combat abilities (backgrounds) for a mix-and-match approach* to building a PC. A D&D ranger, for example, would be a fighter class with the barbarian or traveler background, while the classic D&D fighter would be a fighter class with the military background, a paladin would be a fighter with the religious background, etc.
- The classes are Fighter (weapon wielder), Gadgeteer (arcane devices), Mystic (primal/natural magic), Theurge (divine pacts), Wizard (arcane lore) and Mastermind (doesn’t fight, but helps others fight better). Each class also has a some options that determine which attributes work best for them; ex. a fighter chooses a fighting style (strong, swift or berserker) and fighting focus (daring, tactical or wary) to determine whether they need Strength or Reflexes and Presence, Intellect or Wits (The D&D ranger would be strong or swift+wary, the D&D barbarian is berserker+daring, and the classic D&D fighter would be strong+tactical).
- Each class also chooses a path that determines how they approach combat; ex. wizards can choose abjurer (defense), benedictor (buffs), interdictor (control), maledictor (damage) or summoner (pets). Non-combat magic is part of the background.
- Backgrounds provide all the non-combat abilities, including skills. They’re designed to be fairly generic; arcanist, aristocrat, artisan, barbarian, commoner, entertainer, military, outlaw, religious, and traveler; with skills, languages and a selection of benefits they pick from as they level up (ex. the arcanist background is where you’ll find utility spells, but also can grant apprentices, familiars and special skill bonuses for learning languages, lore checks and performing rituals).
- Companions, hirelings and mercenaries, plus mass combat rules are built right into the system from the ground up (vs. needing to do conversions from the normal stats into the mass combat stats)... which was actually the original impetus for the tighter “bounded accuracy” on combat mentioned above.
- Lots of optional rules to help the GM tailor the mechanics to his liking (ex. default is “big damn heroes” where PCs start about equal to a 4th level D&D character, but there are options for starting anywhere from level 0 all the way down to level -3; the latter of which is basically equal to a 0-level D&D character).
- Focus on tools for new GMs under the idea that not everyone is going to be starting with WoketC’s games (ex. established GMs don’t need help setting up a campaign region for a sandbox campaign, but a new one could use some guidance).
- In general, put all the options into the core rather than leaving things to later supplements. Ideally a player book with a relatively low price-point should be all a player will ever need to play; supplements would be world books with locations, NPCs and monsters/traps/hazards built using the rules supplied in the core (basically paying for the convenience of not having to build them yourself).