Unlike Mearls and crew, I would not try to re-invent the wheel and would focus on iterating 3e and 4e into a next edition that borrowed from and improved on both. Unlike Mearls and Crew, I would tell the fans of 1e and 2e to fuck the hell off and get out of their grandkids' basements - that shit is just creepy. You might think that's bad marketing, but I'm pretty sure that fans of 1e and 2e AD&D are not actually going to buy new editions of D&D anyways, and telling them off is probably worth points with the people who will. Any "nostalgia" mechanics would be limited to flavor swiped from OD&D, and reprinting to d20 vs 3d6 bell curve charts.
The core book would have 7-12 races - but the races would draw much more heavily from more recent fantasy sources than from Tolkien. The core book would have 12-15 classes, with names cribbed from earlier editions of D&D. The classes would be sorted into some sort of related groups that would look vaguely like 4e "roles" but this would be just to help new players learn via chunking and not really expected to matter.
Then going down my list of 3e/4e comparisons for nitty-gritty bits
- The power schedule would be between late 3.5 and 4e. Vancian casting would be gone, but not all classes would get the exact same power usage schedule.
- Characters would have the much softer death threshholds of 4e
- The Bloodied status (half max HP) is retained as a key building block for triggering effects
- Serious effort should be put into standardizing durations. Most things on duration timers should last at least the 5 minutes for an encounter -- if something is too hosey to last that long then it belongs on a "save ends" timer, not a duration timer. Most things on durations should last a day or less -- week,month, season or year durations will be longer than many campaigns and can mostly be replaced with "until dispelled"
- The range of bows and ranged spells should be greater than 4e's "you get one shot before they are in charge range" but less than 3e's "to scale, you can fireball them while their mini is out the door, across the street and down the hill"
- scaling ??
- The 4e idea of weapon proficiencies provide a bonus is retained over the pre-4e idea of non-proficiency penalties. However unlike 4e, all weapons have the same proficiency bonus. (the other option would be to apply proficiency to damage but not to-hit)
- Skill DCs for each and every skill need to come in both static and opposed varieties. DMs should be encouraged to adjudicate unusual uses of the skill by comparing it to an existing static DC chart.
- All skills need to have a described combat use. Stealth sets up sneak attack damage, Perception prevents that setup, Bluff allows for Feinting, Intimidate allows imposing Morale penalties, Ride allows faster combat movement/better charges, Knowledge skills need to open up specific counter moves (like knowing to hold an alligator's jaw shut)
- Critical hits need to be more standard and less stupid that both 3e and 4e. They should have a confirmation roll and a confirmed crit should deal max damage plus a predefined status ailment. By max damage, I mean that all damage components of the attack are maximum, and no additional non-maximized random damage components are added. This status ailment needs to be universal to all crits with all weapons against all opponents.
- The action sequence would be 4e's Move, Minor, Standard, can trade action types down. Out-of-turn actions need to be more streamlined than both 3e and 4e, collapsing 3e's immediate action and attacks of opportunity as well as 4e's immediate reaction, immediate reaction, and opportunity action into a single out-of-turn action per go-round.
- All combat powers would be on one of the following use schedules: *
- At-Will (and most PC at wills have a few options in the style of Divine Power domain feats)
- Encounter (usable once per 5 min)
- Any single power from this set is usable once per Encounter (5 min recharge for whole group)
- Takes an action to Recharge (type of action can vary)
- The power progression would be between 3e and 4e. As they leveled up, characters would get improvements to old powers and more frequent use of old powers (similar to 3e); however characters would also get options to swap out low-level powers for higher level powers (similar to 4e)
- Status ailments need to be easier and less obscure to cure than 3e. Having all of five spells than cure Insanity is in fact insane. They also need to harder and slightly more specific to cure than 4e. Being able to cure anything that isn't death or petrification by waiting 5 minutes strains suspension of disbelief. I would suggest 3 tiers of ailments, cured by 3 tiers of powers.
- The game would be closer to 3e's wealth-by-level than 4e's "letters to santa / we have no idea", however it would be stressed than WBL is a suggestion and roughly correlates to the random treasure tables. Care would be taken that characters are capable of facing level appropriate opposition without any particular equipment, and any class that was expected to fight with swords would get abilties like "make sword count as +X" at the same time monsters with "only hit by +X or better weapons" show up.
- Also, the idea of equipment providing numeric bonuses that influence the combat RNG probably needs to go. If +X swords are mandatory for D&D, they can add +X to damage, or +X additional uses of powers from the "any one of these per encounter" pool or something else that isn't on the d20 roll..
That's not anywhere near a complete outline, and that's only 45 minutes, but that's all the time I have for now.