I imagine a suggestion that deviates from 5e is going to fall on deaf ears, but I really hate the way multiclassing works. The way class features are handled is fine, but the way proficiencies are done is just stupid. It's basically this table I hate:

Multiclassing Proficiencies
Class Proficiencies Gained
Barbarian Shields, simple weapons, martial weapons
Bard Light armor, one skill of your choice, one musical instrument of your choice
Cleric Light armor, medium armor, shields
Druid Light armor, medium armor, shields (druids will not wear armor or use shields made of metal)
Fighter Light armor, medium armor, shields, simple weapons, martial weapons
Monk Simple weapons, shortswords
Paladin Light armor, medium armor, shields, simple weapons, martial weapons
Ranger Light armor, medium armor, shields, simple weapons, martial weapons, one skill from the class's skill list
Rogue Light armor, one skill from the class's skill list, thieves' tools
Sorcerer -
Warlock Light armor, simple weapons
Wizard -

Any result that leads you to ask the question "Should I respec my Wizard 1/Rogue 1 to a Rogue 1/Wizard 1?" means multiclassing isn't doing what it is supposed to do. You should get exactly the same thing when you take a Level of Rogue first as when you take a Level of Wizard first. Right now, there is a right answer and a wrong answer.

I think this should be changed to basically be like the old 3.5e gestalt rules, but for 1st Level. You get your best hp at 1st Level and other hp at other levels, your highest number of starting proficiencies for any class, and all armor and weapon proficiencies of any class, and if you are a Rogue or Bard, you should get Expertise x2 (but not Expertise x4 if you are a Rogue/Bard). The only real nerf would be if you go Rogue/Bard or Rogue/Ranger, you wouldn't pick up that additional proficiency, but that is only fair. Then you could get rid of the table and players could multiclass without worrying about the correct order.

Another thing I think Larian should do is allow something to happen after 12th Level. I heard someone estimate there is enough xp to get you to maybe 15th Level. That's 3 additional levels where you aren't advancing. I propose Larian should allow you to pick up Gestalt Levels at each "Level" after 12th. So if you were a Rogue 12, you could gain Fighter 1, but still be a 12th Level character. That would make the late game more interesting without dramatically impacting power. Also, if Larian ever gets around to more granular difficulty settings, you could potentially set a "gestalt" slider to lower than 12, which means your character could only advance to 10th or 11th Level, but would still gain the benefits of Gestalt Levels after you top out. (This would be for anyone who thinks it would be too easy with Gestalt Levels.) A 10th Level Fighter 10/Rogue 5 isn't going to be dramatically better than a 12th Level Fighter, for example, but at least you would have the fun of continuing to advance until the end of the game.