Cervantes wrote:The "slow advancement" thing isn't a premise, sorry, it's more like what the system forces on you because of bounded accuracy. As in, level 5 to level 10 is not that much of a power jump.
Well... kind of. Martials are just maxing out their stats (and getting a fuckton more HP), but casters are getting a lot of good shit over those 5 levels. The teens are much more problematic for progression since 6th-9th are 1/day each only, go fuck yourself.
The real problem for 5e in terms of advancement is saving throws and other defenses. For most of your saving throws, you start at zero and stay at zero, and there is jack/shit you can do about, and they odds of running into a screw you encounter go up and up.
From what I gather, 5e doesn't really do high fantasy that well because at the highest level you're still going to get fucking massacred by enough level 1 mooks. So either you're going to be middling in a world with archmages you can't dare fuck with (ugh) or the archmages aren't that strong. But the rules definitely want to be high fantasy, you're right. Ugh again.
Well, none of that really has anything to do with high fantasy. Except that there are potentially lots of archmages running around, which does make it high fantasy. People are talking to gods and getting answers (or miracles), barbarians can summon animals to demand answers about nearby things or creatures, and most of the classes have the option to throw spells around at fairly low level. And it isn't even weird by default. But...
Encounter design for 5e is simply that big fights shouldn't happen ever. Yes hobgoblins will fucking murder the party, so fighting hobgoblin tribes is simply not a part of the game. They're lieutenants for goblins or some shit.
Seriously, 5e has a multiplier for the number of creatures involved in an encounter. It starts at
two creatures. A medium encounter for 4 1st level characters involves creatures worth 200 xp. This is two CR 1/2 creatures. But the multiplier for 2 creatures is 1.5, which puts it at 300 (virtual) XP (the multiplier doesn't count for what they actually get), which makes it a hard encounter.
3-6 creatures is a x2 multiplier, 7-10 is x2.5, 11-14 is x3 and 15+ is x4. So using fractional CR creatures in encounters staggeringly hard to do, because the multipliers make them seem ridiculously challenging even when they're not, and adjusting on the fly depends a lot on map position and available spells (clustered, they're just an AoE spell waiting to happen, unless they have high HP, which a lot of them do)
16 Kobolds (CR 1/8, so 400 xp for the lot), should be a medium encounter for four 2nd level characters. Supposedly, with the multiplier, they should be worth 1600 XP, which is a deadly encounter for 3rd level characters, or something between an easy and medium encounter for 5th level characters.
Needless to say, this is a complete mess of shit and nonsense- the system really wants a dogpile on single creature with a CR equal to the party's level. Even at equal numbers, the system wobbles a hell of a lot.
As for the archmage, out of the book, this is a damn stupid fight. Its CR 12, but has 9th level spells, but they're largely shit. Time stop and a giant pile of defensive spells (but concentration says fuck you to casting a giant pile of defensive spells), offensively it has fire bolt, magic missile, lightning bolt and cone of cold and that is seriously it. It can turn invisible at will, so it becomes a question of whether you can blow through its 99 hp before it teleports out, and how many AoEs is can get off before it runs. (Which shouldn't be many- 99 hp at AC15 isn't that hard for a level 12 party, even in 5e- a fighter should be doing about half that with a +2 greatsword and 3 attacks. With action surge, might even kill it outright)
Now, if you swap the spells around, this can absolutely be more dangerous. But there is very little that can be done to one shot 12th level PCs. Feeblemind can absolutely screw people, but finger of death and like is generally soakable (and getting back up is generally someone's bonus action).
The bonus question was mostly a question about "how ought one use math". Because I like math and I like using it but I'm not entirely sure how to go about applying it to game design.
Consistently would probably be the best answer, but the system and setting also have to be interesting, which is why a lot of 70s/80s RPGs failed miserably.
A lot of it depends on where you want to set your standards. 2nd edition had a lot of problems, but encounters were faster and bigger. That pretty much gets lost with fractional CR critters that have to be hacked at with a greatsword multiple times to go down.
My big math bugbear with the current versions of D&D (including pathfinder/starfinder) is hit points are stupidly bloated, and the designers seem to have no concept of how it affects things. Both in terms of drawn out and boring fights (bullet sponge boss fights), and the inability to throw more than a handful of monsters at the party at any one time or the whole thing ends in a TPK against trash mobs.
This is especially true in 5e, because there simply aren't options beyond hack or burn through (generally) large hit point tallies, and scaling is a thing that largely only affects cantrips and sneak attack (and there is a hard limit of 1 sneak attack per round, and rogues don't get multiple attacks beyond TWF, which is ass in 5e).