there is an opportunity to really focus on a basic 5e primer.
Ok, so we're back to what I was hoping B&B would be. Good. Then one thing I want to stress (where B&B deviated) was there can be
nothing new in this project which isn't in 5E already to be a true primer IMO.
In other words,
a system based on 5E that only removes or simplies from 5E, doesn't add anything new.
If you change how systems in 5E work or add new things, this is no longer stripping 5E to its core. So, for this concept, while many of the suggestions upthread are great ones, they are changing or adding, not stripping down to core.
NOTE: all of the following is for 2014 5E. I don't embrace 2024 5E so you won't find references to things like weapon masteries below.
ABILITY SCORES & MODIFIERS
Removing ability scores and just keeping modifiers is understandable. However, since this is a primer and 5E uses the scores, would it be confusing to go from this primer to regular 5E? What is so difficult about the scores you can't keep them?
Well, for one thing, how about ASIs? The biggest confusion IME with scores and modifiers are with ASIs because a +2 ASI translates into a +1 modifier increase. I'll give you a real-life example: a player had an Elf a few years ago and applied the +2 Dexterity ASI. His DEX 16 with +3 modifier became DEX 16 with +5 modifier. In other words, he added the +2 ASI to the modifier--not the score! I've seen this happen a few times, actually.
Another issue (less important) is the +1 ASI, since it only results in a modifier change if the score is odd to begin with. But, if you remove scores and just have modifiers, what does the +1 ASI do? Flip a coin to see if it increases the modifier or not??
RACES
The races are fine, but perhaps limit the number of options? You have to decide how to handle the ASI issue above.
Darkvision is also an issue. Just how it works, and the impact of dim light with perception, etc. can cause problems. I don't know if this is enough to remove it or not.
CLASSES & SUBCLASSES
Classes. In this respect, I would stick to "four core classic" classes: cleric, fighter, rogue, wizard. Do not have generic classes which incorporate these classes and others (like barbarian, ranger, druid, etc.). I see the appeal of a system like Shadowdark (one of my favorites as far as the class structure goes). The additional 5E classes can be mentioned, certainly, but I'd leave it at that.
Now, you could go with other classes instead of these of course. But again, stick with classes that are 5E and strip them down if needed.
Subclasses. As for subclasses, don't use them. While they help a player flesh-out their concept, they add more features which complicate what actions their PC can do on their turn.
HIT POINTS
Max at 1st level and average per level thereafter--no rolling.
SKILLS & PROFICIENCIES
Offer groups of skills based on theme or concept for skills as suggestions. I would consider combining Perception and Investigation into one skill to avoid the confusion that occurs sometimes which skill to use when.
There is no point in confusing things by combining the saves into three a la 3rd edition. The six work fine IME.
This LARGEST issue with this is the proficiency bonus. I often have players want to add proficiency bonus to damage as well as attacks. Also, sometimes players don't add proficiency bonus to saves they with which they have proficiency.
If you follow the four class option above, I would include precalculated saves in the class tables.
EQUIPMENT
Keep choices limited and basic if you want, but this isn't an issue for most players IMO. Do not bother with encumbrance and ignore all reference to electrum pieces.
Weapon properties are a bit trickier.
ADVENTURING
Light. Get rid of dim light. You are either in light or darkness. This would remove the darkvision issue (you can see in darkness--keep it simple).
Food & Water. Get rid of it. Assume characters can find or have food most of the time. Compared to how much gold they get adventuring, the cost of food is a nuisance. You can keep the rules on dehydration and starving in desolate wilderness which the DM can enforce when it happens.
Resting. Specifiy two short rest per long rest. It is what most groups use IME.
Make a long rest "sleep" like it was meant to be (not a "nap", but actual sleep with the 6 hour minimum). Also, simply or solidify the rules on what is considered an interruption. It can be as simple as any interruption, or a bit more complex, but I would suggest removing the 1-hour aspect of it. You have two hours during a long rest to do "other activities", so having to spend an hour or a bit more shouldn't really "reset" the long rest IMO. Perhaps taking any damage? I don't know, you'd have to consider the aspects of it.
COMBAT & ACTIONS
Initiative. Side initiative
is a variant option in 2014 so I would use it. You could still default to DEX to determine the basic order PCs then go in, but freely allow them to change to increase their tactical development for the game maybe?
Natural 20s & 1s. Critical hits and missing on a 1 are fine, so keep them as is. Players seem to grasp the concepts easily enough.
Instant Death. Remove instant death from the primer. IME it only happens within the first few levels and after that most PCs have a bag of hp large enough you will not likely ever encounter overflow topping the max hp.
Bonus Actions. Another tricky mechanic. Some players have issues with them, but IME most don't. Certain classes require bonus actions for key features (like Rogue with Cunning Action). Two-Weapon Fighting also requires bonus actions and is popular. So, without rewriting things you can't remove them--but if you rewrite things you could easily fall outside the "primer" concept.
FEATS
As popular as they are, don't use them in the primer. They adjust the power-dynamic of the game and like subclasses, just add more features resulting in more decision points during the game on what to do or use.
Anyway, I guess that is more than enough for now and I'm sure you get the idea of what I am driving at.