I came to think about some situations where it would be almost impossible to miss a melee spell attack (target is grappling you or vice versa, target is holding you or is close to you, your character already has it's hand on the target, etc.). How do you manage those situations?
As it happens, every spell that involves a melee spell attack also requires a somatic component. From the Player's Handbook:
Spellcasting gestures might include a forceful gesticulation or an intricate set of gestures. If a spell requires a somatic component, the caster must have free use of at least one hand to perform these gestures.
In order to cast these spells, your character would have to release the target with one hand, make their gestures, and then make the touch attack. Depending on the spell in question, the somatic component could be the touch attack itself, but regardless you cannot just hold a target with both hands and then cast the spell.
If you would like to know how to arbitrate circumstances that may grant a caster advantage on their melee spell attack, I'd invite you to look at the various conditions for an idea of what to expect. A brief summary of the conditions from the PHB to help you along:
Your Target is Blinded- Provides advantage on the attack rolls.
Your Target is Grappled (whether by you or someone else)- Does not provide advantage on attack rolls.
You are Invisible- Provides advantage on the attack rolls.
Your Target is Paralyzed, Petrified, or Prone- Provides advantage on the attack rolls.
Your Target is Restrained (whether by you or someone else)- Provides advantage on the attack rolls.
Your Target is Stunned- Provides advantage on the attack rolls.
Your Target is Unconscious- Provides advantage on the attack rolls.
One final note is the big exception to this rule. A sorcerer can cast a Subtle Spell, which requires no somatic component- and thus does not require a free hand to cast, so you could indeed cast the spell while holding your target. In such a case, it's the DM's call whether the circumstance provides you with an advantage on the attack roll or whether it automatically succeeds... though since you don't necessarily auto-hit on even an unconscious person, advantage on the roll is the most likely result, and would presumably be worth the Sorcery Point spent to obtain it.
When you cast any spell with "make a melee spell attack against the target" phrase in the description, you have to perform an attack roll. Basically you treat it as an unarmed attack using your spellcasting modifier. It is irrelevant if you have ever touched the target before, primarily for the balance reasons. If this bothers you as being "unrealistic", imagine you have to hit your target in the right spot, use right application of the force, or make a specific gesture beforehand. Keep in mind there are no touch attacks in 5e — armored characters are still more protected, even from some kind of magic.
"But I've successfully touched him last round, can I just don't release him?"
Keep in mind that a round represents 6 seconds (a lot!) in the game world, and a "turn" is nothing but a game mechanic. Turns don't exist in the game world. Despite you can move and take actions "in your turn", characters act simultaneously. They are also aware of their surroundings and act accordingly, even when "it is not their turn":
In a fight, everyone is constantly watching for enemies
(PHB page 195)
Characters don't stay "frozen" between turns. That means, that even if you have ended your turn touching someone, don't expect you will still being touching them next round. This makes the "your character already has their hand on the target" situation implausible.
"Okay, I can't maintain touch so easily. But what if I am grappling him?"
As this answer explains, "grappling" is a situation when you limit a creature's movement. It is a condition in which the creature's speed is reduced to zero. Strictly speaking, you don't have to have a firm grip on the creature to do that. Regardless, even when you hold them tight, that doesn't give you automatic melee spell attack, just like holding your hand one somebody's shoulder doesn't automatically punch them.
Every round takes 6 seconds (on Roll20), with a movement action and a normal action (plus bonus actions). That gives 3 seconds for spell casting. If you were using a spell with a somatic component, which usually take a whole action, then the 3 seconds consists of the character wavy their arms, or whatever the spell requires. If a person wanted to cast this without an attack roll involved, I would have them make a Dexterity (Slight of Hand) check with a DC of the target's passive wisdom score. If it fails, the target sees the hand movement and has a chance to move. The spell caster would make an attack roll. If the check succeeds the spell automatically hits. If it is a verbal spell only, which are usually bonus actions, then the spell can be cast very quickly, without the target being touched having a chance to react.
Otherwise, what you could do is halve or reduce the AC of the target, and then just have a normal attack roll made. My DM does this, but it doesn't make as much sense to me as what I stated above.
