Two more general notes on simple changes that are likely to have widespread effects on the game.
First, one new mechanic replacing many is “proficiency bonus.” This takes the place of what has been many tables across the editions: THAC0, to hit bonuses, saving throw tables, spell DCs, proficiencies, and more. Those were all separate tables, sometimes separate tables by class, sometimes separate table by effect. For example, saving throws in second edition were divided by class, level, and what you are saving against; combine those three factors, look up the right cell on a chart, and there you have your base number to which to apply other modifiers. The same edition
introduced included THAC0, which simplified several tables into one odd number, the number you needed to hit an enemy (modified by all these things), and it meant “To Hit Armor Class 0,” where 0 is not “no armor” but rather “pretty good armor,” as armor classes ranged from +10 to -10. In third edition, your number of attacks per round was a factor of your attack bonus, and each class worked from one of three attack bonus tables (another simplification over time).
Anyway, sweep all of that away for something simpler. If you are proficient in a weapon, you get a bonus. That applies to spellcasters’ focuses to, say a wizard’s want or a bard’s harp. The same bonus also applies to a rogue’s lockpicking tools and a ranger’s tracking. If you are proficient in a type of saving throw, you get the bonus. No separate tables for similar things, just the same bonus for every class, ranging from +2 at the start to +6 at the level cap. And then you apply all those modifiers.
Simplicity is a virtue. I’m concerned that giving spellcasters the same bonus as the fighters only helps them more, but I think they got nerfed on the other end to balance it out. Multiple attacks are now handled through a separate system. You can now have proficiency is just about anything, since that term has become a catch-all for “gets a level-based bonus.”
Second, the economy of actions has again been revised, or at least renamed. You get a move, an action, and potentially a bonus action. That is pretty close to fourth edition, and similar to third if you remove the possibility of a “whole round action.” What has been variously called a minor or swift action is now a “bonus” action, and you get one a turn from whatever menu you can acquire from your class, equipment, whatever. Maybe you are a two-weapon fighter and use the bonus action for off-hand attacks. Maybe you are a rogue and use the bonus action to pick a lock mid-combat, you cunning halfling.
A great merit here is the ability to give someone more cool options without surrendering too much to min-max. The limitations of action + bonus action is a balancer; while someone can acquire many great abilities, there is a built-in speed limit, so you get diversity rather than a master of all trades. Well, maybe you can master all trades, but you can only use so much at once. Preferably with a proficiency bonus and advantage on your side.