I'll try to state my opinion. It's difficult, because it's a "liquid" opinion, even if Multiclassing is one of my top priorities for the game (all my characters have been MCs more or less), and I usually have very clear ideas on game design...
I'm going to say 4e Hybrid is my favorite type of MC, especially when coupled with additional MC feats.
The good thing about 4e Hybrid, that I think could be maintained in 5e, is that you can choose at any level if you want to get your power from class A or class B. The bad thing is that many classes lost too many features and you could buy just one or two with Hybrid Talent, but that's fixable IMO.
I'm a bit worried about "shooting for 3e-style multiclassing", because even if they maintain 4e-style MC-like feats to get some minor features of other classes, if you make the true multiclassing system in the form of "take a level in X", there will always be the problem of having to loose something.
True, the math of saves and attack bonuses is fixable.
True, you should lose something when multiclassing.
But still, I can't see 3e-style multiclassing being as flexible as 4e-style hybridizing where at any given level you go "Ok, should I get a Fighter exploit or a Sorcerer spell?"... If they manage to solve this problem, with a solution that I'd call "Level Equivalence" or something, then I'm ok with 3e-style. But then again, if they do solve it, will they be able to retain the difference between, say, a Fighter 4th/Sorcerer 7th and a Sorcerer 4th/Fighter 7th? My guess is that the level should just put a cap on the
number of different features and available ones, not on the power of each of them. This, for a spellcaster, would mean that your caster level or equivalent mechanic would still be based on your level, and damage of your spells as well. It could also mean that maximum spell level could be also based on level. What you'd loose is number of spell slots, spells known, special features.
Practical example of my ideal 5e multiclassing:
Fighter
1 Basic Maneuvers, Challenge, Favored Weapon
2 Battle Stances
3 Fighter Tactics (Defender, Striker, Controller, Leader)
4 Weapon Specialization, New Maneuver
Features = approx. 2x class level
Sorcerer (
thread about my ideal 5e sorcerer)
1 First Spell-seed, Basic Forms, Arcane Fundamentals
2 Elemental Halo
3 Arcane Surge
4 Second Spell-seed, New Form
Features = approx. 2x class level
Fighter 3 / Sorcerer 1:you get 2 features from either Sorcerer or Fighter, up to level 4.
Choosing Weapon Specialization and Elemental Halo gives same power in melee as Fighter 4 plus additional synergistic feature; choosing First spell-seed and Basic Forms gives nearly full-fledged Sorcerer casting, but a Sorcerer of the same level would have two spell-seeds (basic elements to form spells), and more forms, and the encounter Arcane Surge to cast powerful spells. And you'd loose Weapon Specialization.
-> You always loose something, but you can choose what to loose and what to gain, and the total character level would be the cap.
Now example with Wizard, which is more problematic since it has spell-slots. (There would still be classes that are more or less difficult to multiclass, I guess. Wizard could be a difficult one)
Wizard
1 Cantrips, Basic Spell, Level 1 Spells (2)
2 Level 1 Spells (3), Ritual Casting
3 Level 2 Spells (3/1)
4 Second Basic Spell, Level 2 Spells (4/2)
Now, each slot should be considered a different feature, given the power... or not? Let's see what happens following the previous model: we should get approximately 8 features. But we end up having 10 if we count each slot as different. Let's see the different results with the previous example:
Fighter 3/Wizard 1: you get 2 features from either Fighter or Wizard, up to level 4.
Single Slots are features -> Level 2 Spells (2) [these are two features, maximum level]
Multiple Slots from one level are single features -> Level 2 Spells (0/1), Level 2 Spells (1/2)
So I guess we should count single slots as class features, or you'd end up getting spell slots pretty fast multiclassing as Wizard.
An Ideal Fighter 3/Wizard 1 could get a single Level 2 slot and a Basic Spell (at-will).
Now another problem: class proficiencies. I'd say a single proficiency per level taken into second or third class. Only your first class gets all the proficiencies. What does this mean? That the Fighter 3/Wizard 1 would only learn how to use a Wand, not an Orb, if they're different implements. Or could need to get the "Spellbook proficiency" first, just to be able to use his spell slots. On the other hand, a Wizard 3/Fighter 1 looking for some better armor, would only get Leather armor at first, and would get Chain (or Hide) only when Wizard 3/Fighter 2. Note that he/she would likely advance spell slots of his/her highest level only. So the choice wouldn't limit actual power, but a lack of 1st level spell slots could limit versatility and durability, if wanting to take Fighter features at all (could always want Fighter levels just for HPs and proficiencies...)
EDIT: Another possibility to balance out and decide what features you can take by multiclassing would be giving some things requirements. If we go "Weapon Specialization I" and "Weapon Specialization II", it would mean that to get WS II you'd need WS I first. You could still get both with just one level of Fighter if your total level is right, but you'd have to invest everything on it, you couldn't just jump to II simply because your total level allows it. Same goes for spell slots but always counting just 1 of them at a time. We have "Spells I (3 slots)" and "Spells II (1 slot)" and to take that level 2 slot we'd need to take a level 1 slot too, making the choice a bit more difficult. The simple act of adding numbers to a class feature would imply that to take "number II" you'd need "number I" first. If wanting to be creative, we'd simply add multiclassing requirements under each class feature, and state that the Sorcerer's Arcane Surge needs a Spell-seed and Basic Forms, while the Elemental Halo would require just a Spell-seed. Things as Ritual Casting, Basic Spell or Arcane Fundamentals would be poach-able without requirements...
EDIT 2: To make things really granular and controlled with this multiclassing system, we could also state that you can always only have as many class features in a given class as (class level in that class x 2). You could always retrain in order to keep your power level in row with your character level. A Fighter 6/Wizard 1 could still have access to the spell slot level of a Wizard 7, but it would be only one slot, with the other slot being of the inferior level. The character would have attained this by retraining first level slot into second level and second level into third level and so on. You'd still lack the ability to cast many spells. You'd get that only by advancing Wizard more.
