This is a long one, so buckle up.
I got into a discussion with my boyfriend about games have and have not gotten pet summoning systems right. Some examples that came up off the top of my head on the bad side were Guild Wars 2, Skyrim, and Perfect World, with the good side being WoW and CoH.
In Guild Wars 2, there's three classes that can set themselves up for the utilization of minion creatures; Ranger, Necromancer, and Mesmer. Ranger has a pet stapled to it; to account for this, they have a lower damage than the vast majority of the other classes, and have since the game's launch; there's been wide demand since launch for the ability to just not take a pet for the fact that they had terrible AI, died easily, didn't make up for the damage loss, and being something that vampiric bosses can simply leech from to screw over the party. The new specialization, Soulbeast, can get rid of the pet for personal buffs, and the ranger has been in the best place it's ever been in since that spec was released, which is bad. The necromancer, meanwhile, can choose to fill its skill slots with a wide variety undead minions, all of which are disposable and can be resummoned with a little effort, but are the next best thing to useless anyways in high level content due to the fact that they aren't up to par with what is expected of them. Mesmer minions, their illusions, are even more expendable - you are in fact meant to shatter them on your enemy for damage and utility effects, meaning you build them up naturally through standard combos and then get rid of them all; they're the strongest implementation of minion creatures in the game, and you can't build a minion build with them because of how ludicrously disposable they are. for every one of these builds, you can put a lot into the pet, but it'll amount to very little in the end, and detract from your character's other abilities.
Skyrim (and many of the other Elder Scrolls games) has an entirely different problem. A conjuration specialist mage could choose to basically not take part in vast swathes of the game, due to that fact that if they call up a critter, it'll turn the corner and engage the enemy for you. you lose nothing by doing this tactic either; magicka regenerates fast enough that if you sit out of the fight, any time your summon dies, you can simply summon a new one, and they're reasonably on par with other creatures you can encounter. You send enough of one thing at a creature, and it'll be whittled down over time. Eventually, you could even summon two top-level creatures at a time, allowing you to obviate basically every combat scenario in the game for very little effort.
Perfect World, like skyrim, has the "overpowered pet" problem, but in a different direction. Rather than being infinitely expendable while also being powerful, the pet of the venomancer class is nearly a party's worth of defenses unto itself if you do it right, and with the venomancer character playing healbot to their minion, they could solo dungeons meant for entire groups of players to clear. It became a popular farming strategy, in fact, to the point of being memetic among the game's playerbase, where even if you didn't main a venomancer, you still had one so that you could go farming. Dungeons were nerfed several times to account for venomancer farming runs, and the venomancer was nerfed several times, to no avail. The pet was hard to get back after it died, but it never actually died.
But those are the bad games for this sort of thing. when it comes to WoW, you have two high-end pet systems in place - one for the warlock summoning demons, and one for the beast master hunter and their pet. Demonology warlocks are focused primarily on themselves and their spells, with the demons they call upon playing the role of extra damage and a side of defense, primarily just by putting an extra body between the target and the warlock. the warlock casts the spells that actually do the summoning, including summong extra, low-end demons to send into the fray that are completely expendable, even more so than the main demon pet. Beast Master Hunters, meanwhile, get a single pet that can play tank or damager for them based on the group they're in. It's much like how the venomancer would be if they were actually balanced, as the pet can operate much like the venomancer pet would, but the hunter doesn't have the kit and tools to keep it alive indefinitely, as the hunter itself is primarily a damage dealing class, solving the issue neatly while still allowing the playstyle of the pet being a tank to be available to the hunter. The hunter's pet is valuable, without being expendable.
The big thing that can I glean from this? How balanced a minion creature is is determined primarily how long and how much of your build you need to use up to make a summoned creature viable, how strong the minion is once summoned, and how easy it is to replace once it's no longer available.
- A minion that has too much value for how easily it can be replaced, or how much of a build it takes up will always be overpowered compared to other options. Conversely, a weak minion must have similarly low build requirements and be easy to replace to be worthwhile.
- A minion that is too easy to replace for its value or build requirements will lead to endless waves of minions being thrown at targets, which even if weak will whittle down high-end targets. There's actually not too much problem with making minions difficult to replace, as it makes them a "between combat" replacement deal; it just means the summoner will suffer more strongly from attrition than their peers. Still, one must balance the value and build requirements based on how long it takes to replace them.
- A minion must be as valuable as is proportional to the build costs required for it. A creature that costs too little of the summoner's build to utilize for its value will result in the summoner specializing in other places, allowing the pet to wreck house even while the summoner does their own thing. Conversely, a weak minion that doesn't grow enough with a build dedicated is the next best thing to useless, and can even be a liability in certain scenarios.
I'm fairly certain someone better at math than I am could lay this out as an equation. You must balance these aspects to have minions that feel good, which leads me into my next point: the fact that CoH balanced these values near perfectly through the mastermind structure.
Do you remember your crappy level 1 minion? Easily replaced, as you only needed to use a single power to get it back. Said power had a fairly long cast time, so it still hurt when you lost the minion, but it wasn't difficult to get back at all, making it not very valuable, but easily replaced. The moment it started to feel underwhelming - around level 6 - you got your second tier 1 minion and your first empowerment. your tier 1 summon spell becomes more valuable, but to keep your minions up to snuff either way, you needed to empower them; you've just more than doubled their (admittedly quite low) value, in exchange for doubling the build cost (two slots, rather than one) and casting time to make them combat ready.
And then, at level 12, you got your third minion, and your first tier 2 minion at that. Still capable of being empowered with your original empowerment ability, you're up to a three slot build cost and triple casting time to get everything ready, but life is downright good at the moment, as the minions feel valuable for the level, since the tier 2 minions is out and out strong, especially since it has your two tier 1 minions as backup dancers. And that lasts until about level 18, where your your tier 2 minion isn't the unstoppable badass he was six levels ago, but you get a third tier 1 to pick up the slack, and that's enough to keep you going for another 24, when you get your second tier 2 minions; value boosts, since the old amount of value wasn't keeping anymore for the build and expendability cost it required from you. And then, finally, you got your single tier 3 minion at level; another build requirement, another value boost, and another increase in how long it takes you to replace your minions. finally, you have a high enough value to see you through for quite a number of levels until your second empowerment spell comes along on at level 32, and that will see you all the way to endgame.
This is the important thing; every time that you lagged behind in CoH, it increased your minion value to make up for it. It often requires you to commit parts of your build to it, but it maintains being viable at all levels without being overpowered by making certain to balance the available cost in fairly blunt ways. Minions dying too often? Here's an extra! They're not doing enough? Here, give them new abilities! It even included a few boosts that didn't require expending extra resources from your build or cast times by giving you additional minions now and then.
This is something that I feel needs to be kept in mind when making minions in CoT, for the devs. This is in fact important; every game I can think of that fails at summoning puts the onus on one of these parts of the balance equation.
An infinite number of tries doesn't mean that any one of those tries will succeed. I could flip an infinite number of pennies an infinite number of times and, barring genuine randomness, they will never come up "Waffles".