Running without multi-classing, feat density or another mechanic?

I have, scattered throughout these posts, strongly implied the idea that you more or less kit-build your character.

It started way back in the first post with the idea that for my 6e we'd use the 5e as the essential building block for all characters, and it got a bit further developed scattered around in various posts. In essence, your class gives you quite minimal class features, in 5e, but balances a bit what you can get access to, and your subclasses, how many HP and MP you get, but most importantly what class training you get access to. That class training, there need to be more options than you can actually ever take, lets you define your flavour. So lets say, for the sake of example, there are 5 picks available at every level, in every class (that seems like too many, but we're playing make believe here). You only get to take three of them... so you can opt for tank, more tank, yet more tank, or attack, attack, more attack or something in between and make your fighters have a lot more variety. 

I also said that feats would be an integrated part of 6e, unlike in 5e where they remain optional.

Finally, I said that multi-classing would not be allowed, it would be handled by out-picking from your class.

This gives us two choices, and some maths and balance issues. 

First up, how often do we give out feats? This doesn't have to be the same for all classes, but we have a significant balance here. Lets think about some extremes. If we say 1 feat per level, characters are really defined by their feats almost as much as their features - that's not necessarily bad, but I'm not sure that's where we want to go. If we go 1 feat only, that's pointless. Somewhere around 1 feat per 3 to 5 levels seems like a happier medium to me. That would give you between 4 and 6 feats. The question then becomes, if feats are used to let you purchase what would, effectively become cross-class training, how to keep them balanced but attractive enough in terms of what you get to make it worth taking.

The alternative would be a blanket rule saying you can swap out some of your core 3 picks (from our example above) at certain points to take picks from other classes.

Personally, I'm in favour of the feat approach. You have a string of feats that basically say "you can take feature picks from class X instead of your main class whenever you gain a level." Obviously that needs a bit of tidying up. But it caps how often you can outpick nicely, because if you only get a feat every 4 levels (say) you can only pick a new class to outpick into every 4 levels... I think there needs to be some balance testing here. Do you get a free pick in your new class so the feat balances nicely to the others? Clearly you need to build up the prerequisites as normal. So if you outpick wizard you need to take Pyromancer-1 before you can take Pyromancer-2. I think all the feats need to give you something, so if you take the wizard outpick as a fighter, say, you can pick up MP as part of the deal. Maybe you get n MP to start, and +n every time you take a wizard pick, you also add the +x MP to your skill pick repertoire, that is not normally on the fighter list. That doesn't seem overly strong for a feat (it might be a little weak still, but maybe not) and it lets you dabble without going down the Eldritch Knight route. 

How does this work overall?

Well 5e front-loads character classes, there is a lot to be said, for example, for 1-2 levels of Fighter (fighting style, heavy armour, action surge, second wind), and the same for quite a few other 1-2 level dips (all those proficiencies, bonus action choices and stealth attack for a Rogue, oaths for a Warlock, domains and channel divinity for a Cleric). You delay some top end spells and so on, but most campaigns don't get there, and you can be really strong all the way through the mid-levels where most campaigns peak and end. This does away with that dip mentality, while letting you actually build a character that constructively outpicks from other classes if you wish. If you want a fighter with a little bit of Rage, go for it. A monk with a smidgeon of magic, no problem. 

In this system, say, for example, you want to play a tanky fighter. There will be pick options for some damage mitigation, boosting your resistances, protecting others, maybe even a proper taunt mechanic. Exactly how that will all work we'll wave our hands over and say "it works, it's balanced" in this post. Obviously there's a huge amount of work there, but without hours of play-testing and tweaking getting it right is just not possible. Two obvious things are missing. One is some abjuration magic, the other is a decent bit of healing. Whether you're self-buffing and self-healing, or aiding your comrades in the front line doesn't matter, they're both useful extra features for a tank. They might not be the first and second feats you take, something to boost your shield and armour might get in there, but they might line up as your third and fourth. You don't have to be a super healer, but the ability to get the main party healer back on their feet reliably, or heal yourself a bit, is pretty great in someone, and covering your ally like that fits nicely for a tank IMO. If you're in a striker role, you might want to add something like Magic Missile to your repertoire instead, for those times when you just need to hit. If you're the Cleric, you can pick up an offensive option from Warlock or the Druid can take some skills and offence from Ranger...

Depending on exactly how the feats are worded, stranger combinations become possible. It is unlikely (not impossible, but unlikely) that you'd build a Druid-Paladin in 5e. Druids look at Wis, Paladins at Str, Con and Cha. You might if you roll and do well, a Druid-Oath of Ancients Paladin works thematically after all, and several Druid subclasses don't rely too heavily on wild shape. If you can outpick Paladin from Druid without a Cha limit (or the other way around, with no Wis limit) you could make an interesting hybrid choice. A Druid with heavy armour, smite and lay on hands maybe? A Paladin with the ability to "wild" shape into a celestial or a fiend perhaps? They both sound kind of fun to me, especially the second one, and something that, balance permitting, we ought to let players explore. A Druid taking Paladin spells, assuming they stay Cha-driven, and a Paladin picking Druid spells with their Wis base will still struggle to use a lot of them well. But a Druid burning spells to smite could be fearsome... 

And, although I don't mention the red-haired child that is the Artificer that much, I can imagine out-picking Artificer to make a few potions and magic items might be quite popular!