Why is the conjuror gimped? Why can't he summon creatures?

[Vos]

He can summon a familiar at first level. He gets a willing spirit Fey, Celestial, or Fiendish (his choice). The spirit serves him willingly.

So why can't other spirits serve the caster in a similar way?
say like taking the form of a skeleton, a bandit (from the final entries of the monster manual), or other animals, and fight for the caster.

A willing spirit will not turn on the caster. Concentration will not even be needed. Say a first level spell summons a spirit creature for a minute. Time could increase at higher levels of the casting.

3.5 could summon all sorts of beasties. I see 5e has higher levels where fiends could be summoned, but I don't want to play a caster who would traffic with such evil. Where are the celestials and the fey who could be summoned?
~~sure he can summon elementals, but I'm talking about all levels here, even the lower levels.

Can you summon creatures in Ravinica? I don't have the supplement, but its based off of a summoning card game.

and a bonus question for DMs: do you allow for spell research?

[LordEntrails]

Aren't their like 8 conjure x spells in the PHB? What am I missing? Are those spells not in the spell list you are wanting them to be in? I'm not super familiar with character creation so I'm probably missing some obvious things. :)

As for your bonus question; do I allow spell research? Sure, I guess I would. But I've never found an adventurer who wants to spend 5 or 10 years researching a spell rather than adventuring :) The way I think of it, if it only took a month or two to research and discover a new spell, then every mage would be doing it. Besides, what makes adventurers special is their ability to adventure, not run a business, or be an academic etc.

[Zacchaeus]

First of all this is 5e not 3.5. There is less distinction between the schools of magic and as a conjurer you aren't restricted to a narrow school of spells. The Wizard conjurer is more a conjurer of things (Fog, Tentacles, Daggers) rather than a conjurer of animals or beasts.

Conjuring animals/celestials/fey etc is something Druids or Clerics do rather than Wizards.

As far as Ravnica goes, this is more a setting rather than changing rules. So no, Wizards aren't changed. There's a couple of new archetypes for Cleric and Druid and two new spells. But otherwise it doesn't change things.

[lostsanityreturned]

A willing servant that doesn't need concentration would be a NPC hireling or just a NPC follower.

A DM can arbitrate that but you shouldn't be getting free characters and thus free action economy like that just from a spell.

As it is a conjuration wizard can
- summon elementals
- summon demons
- animated objects
- create / animated undead
- tiny servant / unseen servant
- summon familiar
- phantom steed
- create homonculous (not a conjuration spell but still)
- casting instant summons on a contracted demon's bracer.

And a few more I have likely missed in addition to all the other conjuration abilities that don't result in NPCs.

But your options for summoning should be limited at level 1 and the familiar / unseen servant fills that role imo.

[GavinRuneblade]

In addition to the other comments a conjurer is not gimped, in many ways the various summoning spells are the most efficient use of spell slots for potential damage. And some of the things a conjurer can summon are so powerful that the Adventurer's League had to forbid them. And the various "things" include monstrous tentacles, walls, etc. in addition the stuff lostsanity listed.

[L. R. Ballard]

say like taking the form of a skeleton. . . . Concentration will not even be needed.

Animate Dead is a third-level necromantic wizard spell that allows the caster to create skeletons and zombies from existing bones or corpses. The undead are under the caster's control for 24 hours, no Concentration check needed.