The spell list -really- needs to have a lot of the tradition cut out of it... but.
I don't think control spells are the real issue, there, so much as the structure of them being -so- stridently carried over from previous editions. Take "Hold Person" as an example.
You've got a long duration effect, you lock the target down completely, everyone gets to whup their butt without fear and with crits, etc. It's just -too- packed. And it does it all with a total of 4 mitigating factors:
1) Saving Throw.
A simple binary of "Yes, controlled" or "No, spell ends". There is no scenario in which this is a good design goal. There should always be levels of success to spells like this, perhaps even -gated- by the type of entity you're casting the spell on rather than a yes/no blip. I don't even mean humanoid vs dragon vs beast. I mean normal enemy versus Elite or Legendary or whatever term you wanna use. Make it take partial effect in some cases.
2) End of Turn Saves.
This is awful, in my opinion, on its face. In the best case scenario, the caster and their -entire- party act after the villain and everyone gets to take a full round of swats before they get a chance to save. Worst case? The caster's turn ends, the held target's turn begins, and they save at the end of their turn. All the spell did was go "No" to a single turn.
3) Concentration.
Great in theory, mildly annoying in play. But hey. If you hold the BBEG fighter, at least one of his minions can throw a pebble at you, deal 1 damage, and force you to make a DC 10 Con save or lose your spell. Exists both to make control spells and buff/debuff spells less common and more risky than they already are.
4) Legendary Saves
The great middle finger to any spell whatsoever.
So how do we make them better?
1) Variable Effect
Sure you could use saving throw degrees. But how about having controls -be- several levels of effect that impact different creatures to different degrees and in some different ways? Could base it on Proficiency Bonus difference between you and your target. If their proficiency bonus is 3 or more higher than your own, the spell has no effect, save or otherwise. (A nice 'brown trousers!' moment for the party). Proficiency 1-2 higher it has a lesser effect than full power, but it still works. Proficiency equal or -1 to your own? Totally normal effect of the spell. Proficiency -2 to you? BIGGER EFFECT. Proficiency -3 to you, or greater? You just stop that person from contributing, ever again. Might as well have reduced them to 0hp. No save, they're just -wrecked- by the spell. (Also good for a 'brown trouser' moment if the party tries to punch Strahd at level 1 and he just stops the entire group in their tracks with a dismissive gesture, finishes his monologue, and walks away.)
2) Start of Turn -CONTINUAL- Saves
Rather than the spell simply ending when they make the save on the end of their turn, they can roll at the start to see how badly it effects them. On a successful save, the spell is still in effect, but they can at least act to some degree. Hold Person is also less annoying to the player since, hey, maybe you'll still get to act on your turn instead of being locked out of the game, entirely. But hey, it's still annoying, so how about:
3) Spend your Action to End the Spell
You wanna break free from Hold Person? Spend an action to roll a saving throw that ends it in a dramatic fashion. Break the golden chains that bind you. Tear your way free of the vines and thorns. Overcome the psychic compulsion. Whatever it is, you take the time to specifically break and end it. Dramatic. Flavorful. Players -and- NPCs get to do it, which makes things more interesting.
4) Get Rid of Legendary Saves
You no longer need them.
Is it perfect? No. But it's better than what we've got.