Note:
By "previous editions" RotGrub means 2nd edition/AD&D.
A 2nd level spell to give immunity to b/p/s damage? Wow, I think that's my queue to leave..
When you bring in environmental hazards, not everyone is going to fail. In practice, only one or two party members is likely to fail their save. This reduces the expected cost greatly, from "long rest" to "single spell, cast anytime".I don't know. That's kind of like saying that healing spells remove hit point attrition impact. It still costs a resource, and it still has to be spread over the whole party. I just think the cost of removing an exhaustion level is too high in the game. Coupled with that, there are a lot of optional rules out there that utilize the exhaustion levels in various ways (wound levels spring to mind), and this would be a nice way to interact with them.
Conjure Animals
Conjure Celestial
Conjure Elemental
Conjure Fey
Conjure Minor Elementals
Conjure Woodland Beings
These should be using bonus actions to command the summoned creatures on the caster's turn. The command can still be "Move over there and attack that target!" Also, they shouldn't summon groups. These break the action economy over the knee, and make combat significantly longer.
I would have preferred the "conjure lots of critters" spells to give you one swarm instead--gives you X damage per enemy in the swarm's space, Y additional effect, and size is based on the level of the spell--for mass combat (which I believe is the reason for the spells existence), you just cast it at 8th or above to get a huge swarm. Admittedly that makes conjure animals not so different from insect plague, but the other "conjure lots of critters" spells would be different.
I think the conjure single critter spells are mostly all right. It wouldn't have hurt my feelings to make conjure celestial scale similar to the other spells (or at least go up to CR 6 with a 9th level spell), but since they are giving celestials a lot of utility abilities, it is more of a utility spell then a combat one.
…once done there will be a new set of spells to take their place in the freshly vacated lowest tiers of the power spectrum.

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