It's very hard to translate the concept into gameplay that is both unique and good enough to outweigh what you lose by foregoing the standard RPG mechanics.
Suppose it's a single-character RPG built around necromancy. If the summon limit is low, that will play the same as a party-based RPG with an endless supply of disposable NPCs. That's even the case in Diablo 2: if you could hire multiple mercs, there'd be very little reason to play a necromancer. If the summon limit is high, you end up closer to HOMM (and a complete clusterfuck if each minion has an automatic AI rather than being controlled by the player). Either way you've created a party-based RPG where you can easily reconfigure your party to some extent, but with the itemization and character progression of a single-character RPG. That doesn't sound particularly exciting.
Now consider party-based RPGs. There the summon limit must be necessarily low, otherwise a party of 6 necromancers could trivialize most encounters by spamming tons of shitty low-level summons. But if you can only summon a few minions, it doesn't quite feel like necromancy --- more like playing a rigger in Shadowrun where you can swap drones between battles.
A good necromancy game would have to take some hints from Wizardry 4 by being completely built around that character. And it would have to make you absolutely dependent on your minions and give you very few opportunities to replace them with new troops. That turns your whole party into a resource management problem. Another cool tweak would be corpse quality, where it is in your interest to efficiently take down enemies to be guaranteed a high level summon (that ogre won't do much if you've already chopped off both its arms). That could create some cool trade-offs, like avoiding too many powerful summons like skeleton dragons because their toxic breath attacks attacks only leave worthless piles of ash behind.
There could also be encounters that require you to completely retool your current party --- imagine making it to the boss with a powerful team of stone golems, only to realize that the fight takes place in an swamp where the movement penalties render the golems completely immobile so that you have to abandon them for fast but weak poison maggots or the more powerful swamp sharks, which you can only use in swamps. And once the battle is over, you immediately face another challenge where those golems would have been very useful, but converting weak minions to stronger ones comes at a severe penalty, so now you are looking at a really tough fight that will require smart use of your limited resources.
Along the same lines, dungeon exploration could come with puzzle rooms that require you to switch out minions, possibly weakening your party a great deal. Just imagine the butthurt when the player has to replace their bear squid chimera by a zombie rat to push an inaccessible switch. Or maybe the doors in a dungeon are locked by a special spell that only lets Garbage the Weak pass , so you have to kill the guy and then waste one of the spots in your party on that useless deadweight --- also have fun wasting all your buffs on keeping him alive undead while your quality troopers are going down one after the other. Or, even more dickish, imagine you are forced to solve a puzzle in a no-magic zone without any minions while frantically running away from a giant bat, which you could have easily wiped out with a single lvl1 summon in any other scenario.
And of course there could be more intricate party-internal dynamics. Just because the ogre is undead doesn't mean it won't follow its natural instinct of eating all those delicious corpses you summoned. A lich won't like being bossed around, so you should always be ready for mutiny. And it definitely won't fight together with lowly skeletons and maggots. A Frankenstein monster might be afraid of fire, so don't even think of pairing it with an undead dragon. Undead goblins, however, may happily ride those hellhounds, greatly amplifying their deadliness. And if you make sure your minions don't look too disgusting, you might be able to hire some human mercenaries, which has its own advantages (bye bye giant bat in the no-magic zone) and disadvantages (hmm, those silly humans need sleep, food, and they have this weird instinct to flee rather than defending you to their death).
But as you can see, all of that would require completely rethinking combat, encounter design, level design, and resource management. That's not gonna happen in an industry that's known for its aversion to experimentation, and RPGs are particularly dogmatic in this respect.
tl;dr: There's some good gameplay opportunities here, but the whole game has to be designed from the ground up for that purpose by a very talented and creative team. So it's not gonna happen.