Tinkthank, I think I know how you can get units with Summon Allies! In Dom:PPP, most pretenders that now get automatic units had "Summon ally". Those units still exist, they just aren't available in the pretender choosing screen. If you find few that summon units that are not used anymore, you can easily mod those two and add the former to your nation's recruitables (#reccom).
Atleast Moloch (120, devil), Father of the Serpents (Horned Serpent).
Many uniques also had this ability, atleast Arch Devils and all Elemental Royalties, but I believe all those have been replaced.
Also, you could replace unit(s) that are rarely used, like Draconians and Draconians Chiefs.
Replacing Ermor would give you many summoning spells, and unholy priests without unholy spells might be a possibility if they reanimate Ermorian undead. And you might just change some unit and everything that uses it, like Wolf, Call of the Wild, Howl, Werewolf and Wolfherds. Call of the Wild and/or Howl could either be changed, removed or made national spells, Werewolf and Wolfherd would be two units that can summon the unit that would replace Wolf.
I'm sorry I can't come up with anything else, it might help if I knew what you were doing...
