Right now, convocation is only possible if you specialize into Skeleton mages. However, because convocation is an essential skill to make the entire summoner class feel less clunky as a whole, shoehorning this essential skill into an otherwise irrelevant skill tree just seems inappropriate.
Doing this interferes with certain builds that target specific larger minions by introducing more annoying minions to grapple with, within the unreliable targeting system. For example, if you just wanted to rip blood your golem or abomination, it often targets the wrong target in all the chaos when everything is running around. Therefore, some builds might choose to not introduce extra skeletal mages, instead running full golems for example.
The problem here is now the playstyle becomes annoying clunky and you constantly have to wait for your golems to catch up after you use a travel skill.
There are three suggestions from me for this.
Either 1. Implement convocation into the transplant skill tree instead. Convocation and Travel skills should go hand in hand simply because it would otherwise be very annoying to otherwise wait for minions to catch up, so this makes sense.
When the maximum number of say, Abominations, is summoned, change the function of the summon skill to convocate the minions. The current function of resummoning would otherwise be fine, if not for the fact it removes buff skills with a long cooldown like dreadshade every time you want to use transplant and convocate.
Implement some sort of minion target lock system. Having a big minion army with units hasted and with high movespeed is extremely chaotic and expecting players to be able to target focus particular units in all that mess is unreasonable. For skills like dreadshade which are casted once in awhile, it may still be palatable. However, if a player wanted to constantly cast rip blood on a particular minion, it becomes extremely frustrating.