It may be intentional that
planar ally is a cleric spell. I like the idea of a wizard and cleric having to cooperate to pull off the one-two punch of
planar ally and
planar binding. That's how cults get started in the real world, isn't it?
Actually, clerics get Planar Binding too. As do druids and, oddly enough, the non-summoning bard (I suppose they could Magic Secret the Conjure spells!). But, anyways, Conjure class spells last an hour, and it takes an hour to cast Planar Binding. I think the intent is for the classes with summon magic to be able to bind the thing they Conjure.
We do need a Conjure Fiend and Conjure Minor Fiend spell. I don't know if its in the DMG with the Oathbreaker and Death Cleric, since its pretty much the definition of evil magic, especially for Fiend Warlocks. Actually, I'm a bit surprised warlocks don't have access to some kind of Planar Binding magic, even though they get Magic Circle. Oversight?
After that, the question becomes, what do you want to call? Archons will be a celestial. Eladrin from Arborea are now either Fey or Celestials. Modrons? Those are constructs which, if the memory serves, can be created via a magic item in book form. Same with pretty much every kind of undead and Animate/Create Undead class spells. If you want to talk to them, cast Contact Other Plane; if you want servants, you build them.
Slaadi? Those are abominations now, and I don't know of any way to actively deal with aberration of any kind, from aboliths to mind flayers to slaadi. Can't even call them with Planar Ally. Granted, its probably suicidal in the first place to call or create them, or any aberration really.
Animal Lords are probably going to be celestials or fey. The Gith (both races) are traditionally not summons. Ant people probably will be summoned fey or beasts, summoned by the respective spells.
I can't think of anyone that would be left to call after those. Celestial hits almost all the good-aligned planes. Fiend hits all the lower planes. Elementals hits the inner planes. Fey and Woodland hits a lot of the miscellaneous critters, and the rest you can make yourself.
Of all the creatures out there, only the Slaadi and the two Gith factions aren't summoned or made.