When people say that 4e's presentation is not the issue it its something "more", I think that is underselling the power and importance of presentation. The difference between a bad and a good game is not always the rigor of the mechanics....but how well those mechanics tie into a specific narrative, aka do the rules enhance a player's experience or detract?
I personally do think 4e got a lot mechanically right (certainly not all), but I do think the style and narrative is a good bit displaced from historic dnd and that displacement caused a lot of strife with certain groups. It is not the sole issue with 4e, but I think its an important one.
Anyway back to the topic at hand, what can still be mined from 4e that would be useful to include in a 5.5?
- Bloodied Condition: Simple and beautiful. A great condition to let players track their progress in a narrative way. Was a great mechanic to "change the fight" midway through, either by some monsters becoming stronger, weaker, or just changing their behavior.
Saving Throw Duration Tracker: 4e's "saving throw" is actually a fine idea, its just a terrible name because it caused narrative dissonance from every edition of dnd prior to it (because its NOT a saving throw!). But the idea that specific effects have a duration that is maintained by a coin flip, rather than continuously making saves, is perfectly fine and good. 5e kept it with the death save rule, and no reason it can't be adapted for other purposes.
- Action Point: Maintained a bit in the fighter's "action surge", there is a place here with the inspiration system. Call it "greater inspiration", the ability to take an extra action through the spending of said point. Its fun and meaty.
- Minions: One of the best rules in 4e, the ability to just litter the board with throw away bad guys. This is a rule that completes an extremely common narrative in heroic fantasy, the hero's just kicking the crap out of 20 guys before fighting the boss.
- Running: 4e had a very simple run rule. You gained 10 feet to your speed in a round in exchange for a -2 defense. As 5e doesn't have a full run this is one way to take it up one more notch. Perhaps a bonus action that gives you +10 speed and disadvantage on attacks (and perhaps requires dash in order to use). Combined with dash we get a little bit closer to the old 3e run speed but without all of the other rules baggage.
- Monster Classifications: Soldier, Brute, Artillery, Controller. These were very gamey names but it didn't matter because they weren't for the PCs, they were names that immediately told the DM how certain monsters behaved and what purpose they serve in a combat.
- Monster Design: This is a big umbrella, but in a nutshell 4e had an approach to monster design that I believe is superior to 5e's. We can look at a few sub-categories.
- 4e's understood that the best way to fight a party is with a party. It created classifications and tools to quickly put together a group of monsters as a party and use them against your PCs.
- Distinctive Monster abilities. Many 4e monsters just had really cool abilities that were distinctly their own. One example is the lowly kobold, who had the ability to do an extra "5 foot step" in a round (for those who don't remember old 3e mechanics, this was a small move that let you avoid OAs). This ability made kobolds super slippery, they could dart in and out of combat and it was hard to actual finish one of them. Fighting kobolds was a distinctive, memorable encounter, not a "bag of hitpoints" that is the common critique of 5e. I bold this one because I think its the most important philosophy they can readapt from 4th.
- Real Bosses. 5e adopted a philosophy that monsters don't scale up too quickly, allowing for lower level parties to still deal with higher level threats. I personally think they went too far in this direction, and now it can be quite challenging to balance a real "boss fight" that doesn't end with a whimper. Just look at the recent Epic Monster thread on this forum and you can see just how crazy high a CR it takes to "fully challenge a party". While 4e solo monsters took several iterations to get right (the MMI solos were a sloggy mess)....eventually they crafted some really fun and powerful solo creatures that truly created the notion of "boss fight". I want some of that back in 5e.
- The Statblock is for combat. When people talk about 5e spellcasters, this is a part of what they are talking about. I don't care that my lich can cast identify. Heck I barely care what their 1st level spells are when I'm throwing 9th level ones. I want a statblock that is used for one purpose..... a fight. 4e focused its statblocks on the essentials, giving you what was necessary for the creature to fight.... and then put all of the other stuff into narrative text so you could see how the monster works off the clock. I think 4e was too light on the flavorful narrative elements, but that is an issue with the surrounding text, the statblock itself they got right.
To balance that, something I personally
do not want to see come back.... Skill Challenges. Well....kind of.
I have invested a lot into skill challenges, more than most. I actually wrote the book on them.... hehe ok ok I wrote A book called the Obsidian Skill Challenge system, where I broke down the original broken math and rebuilt a new system. But suffice it to say I have invested a lot of time and energy into making skill challenges good.
What I found after all that experience was Skill Challenges can be great as a tailored experience, but are terrible as a general one. A standard skill challenge broke down into "DM describes an event, players try to rationalize why they get to use cool skill X, they roll a bunch of die and either win or lose". Its one of the worst cases of 4e mechanics getting in the way of the narrative, they feel EXTREMELY artificial when used this way.
However, I think you could build certain subsystems off of the chassis. For example, I think you could make a solid chase system using the skill challenge concept..... and chases are one of those things that happens so often in dnd tropes but often doesn't get a good rules treatment. I also think multiple checks to effect something works create in combat..... "combat skill challenges" I think are a lot of fun, and so a subsystem of that could be great.