Buffing PWK is a tricky proposition. It's one of those spells that's terrible in the players' hands, but it's cheap as hell when used by enemy mages.Power word kill could be upped a bit, although it is also quite ok the way it is.
Maybe it should kill off up to 300hp or up to 500hp but I do not know.
I would keep concentration on Stoneskin but I do like all the other changes here.I'd be tempted to remove concentration from stone skin and return it to blocking X number of hits (originally it was attacks, hit or miss but I think reducing damage from actual hits would be better since it is no longer immunity). Maybe something like the following:
Duration: 1 hour or special.
Stone grants resistance to bludgeoning, piercing, and slashing damage. It lasts for the duration or until it has reduced damage from number of attacks equal to 1d4 + 4.
At higher levels. Increase the number of blocked attacks by 1 for each spell slot level above 4th.
This would make it work more or less like the 2e version of the spell which increased the number of absorbed attacks by 1 for every 2 levels of the spellcaster.
Yes, as it is, saving to end "Save-or-Suck" spells every round makes them essentially worthless. Completely agree with you the duration needs to be extended.The concentration requirement would come off quite a few spells.
AoE damage spells such as Fireball, Lightning Bolt, Cone of Cold, etc. would be uncapped at dx (usually d6) per caster level (but you'd have to roll to aim them). With everything in 5e tending to have gobs of h.p. this doesn't seem like a problem.
Fireball would expand to fill the required volume. (this is kind of both a nerf and an un-nerf at the same time)
Save-or-suck - a lot of these would get some duration back before the target would start getting any per-round saves to shrug off. (e.g. Hold Person duration 4 rounds + 1 round / 2 levels, save each round after the 4th round to negate early)
You say a word of power and pick a target within range.Power word kill could be upped a bit, although it is also quite ok the way it is.
Maybe it should kill off up to 300hp or up to 500hp but I do not know.
It's not true that lower-level spells are more accurate to make up for their reduced damage in 5E, though. In 3E, the difference in save DC between a level 3 spell and a level 7 spell was often outside the scope of the d20; they were going to fail, regardless.To people who want damage spells to auto-scale damage like in previous editions (namely 3e): You do realize that in 3e the DC was fixed, right? There was the Heighten Spell metamagic to increase the DC of a spell in 3e, but that was it. In 5e, it's DCs that scale automatically with level (well, proficiency bonus), so 5e just switched it around. In any event, having both damage AND DC auto-scale doesn't strike me as a good idea.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.