Under my criteria, an ASI improvement and another additional attack are not bad but are not interesting or flexible.
Well, I'll give you "not interesting", and technically I guess you are correct - things that are always on are not 'flexible'. But what flexibility would you want? The option of turning them off? The Barbarian is getting +2 to hit and damage on
every attack, +2 to
every STR check, and +2 to
every CON save (CON checks and STR saves, too, but those are less interesting). The fighter gets a 33% boost to his attack rate
all the time.
Contrast those with the rogue being able to turn a failure into a success
once per short rest. The real luck would be in having something critical enough come up to make this matter.
Where the Wizards way out shine them is that each player gets to pick their own spells so that Capstone actually has more flexibility for combat, optimization, and story then all of the different paladin capstones together. One could argue that players could select different wizard spells but usually pick the same ones but then you could also say that most people play the same favored paladin sub-classes so you may only see a couple of different capstones not all of them. Due to variation Wizard schools and spell selects per player the spells selected for the Wizard lvl 18 and 20 abilities actually has greater potential for variation of play than the sum of all the paladin capstones its just a matter of the type of players you have that determine if they make use of different paladin domains or select different spells per wizard school/spell selections or if despite the options they always go the same route. The point is the options are there not weather they take them.
Ok, there is a choice to be made, so I guess you can count that as injecting some flavor. But the signature spells have to be chosen from those already in the wizard's spellbook. So they just get to do the same thing they were doing before, but somewhat more often. Basically, they are just getting two restricted 3rd level spell slots that recharge on a short rest. They could cast the spells before and they could recharge spell slots on a short rest before (Arcane Recovery). Now they can do it somewhat more / more often.
In terms of potency, at level 20, a Wizard has 22 spell slots + 2-10 recharged via Arcane Recovery; this is sort of like getting 6 more slots, a 20-25% increase. (If you look at it using spell points, it's 133 + 15-20 via Arcane Recovery; the capstone adding 30 is ~ 20% increase.) But those slots are restricted to those two spells cast at 3rd level, so the actual impact can be, situationally, less, maybe much less. So, not bad, but I wouldn't call it great either.
On a similar note, Cleric's Divine Intervention is the most open ability in the game (well other than WISH), its real flaw is that it is so open that some players will not know what to do with it.
The 7 day cooldown and DM dependence makes it hard to compare with the others. It definitely has the potential to be flavorful and high-impact; no guarantees, though.
Druids, is open to many forms but they typically will pick the same optimal choses like Giant Eagle. But owl is an option... and becoming a rhino might not be optimal but its an interesting way to open a door and when its unlimited... why not? Also... Unlimited elemental forms?
Your point about utility is valid, but for the most part I think this becomes about having potentially infinite HP, which is a pretty good buff. I have a bit of a bias on this one because I am quite displeased with shapechanging having become the defining trait of the Druid class.
). I understand what you mean when you say "the other 8 are useful all the time", but there's still a distinction among those 8 as to how much of the time they are usable:
