One thing I think is kinda cool about AD&D is human dual classing. You start out as one class, but switch to another. I like it better in concept than demihuman multiclassing. Of course, Gary was never one to allow cool stuff like that to be easy, so AD&D is full of fiddly limitations about ability scores and how you can't rely on your old class powers if you want to gain XP in the new class, etc. Frank Mentzer's Master Set introduces the idea for one of the paths to immortality (the Polymath) but gives no rules on how to make it work.
And I'm playing Final Fantasy III (for the first time ever) on my phone NES emulator, which involves switching classes any time it's convenient, so class switching is on my mind.
A while back I made a streamlined multiclass system for TSR, where I just totaled XP from both classes into one progression chart, instead of the division needed by AD&D's system.
But now I'm thinking of scrapping that idea and going with a modified dual class system for all races in the game. At the moment (I just came up with this idea this morning, so I still need to consider ramifications) I'm thinking it would work like this:
Start as Class A.
When you gain enough XP to attain 4th level in that class, you can instead choose to dual class.
If you dual class, you retain all previous class abilities, but start out in the new class with 0 XP (keep the old XP total noted, though). You immediately gain the abilities of a 1st level PC in the new class, except you must roll for 1st level hit points (my house rule is max at 1st level).
From now on, you progress as your new class. You can freely use abilities from the old class without XP reductions. You can only dual class once.
Energy drain attacks reduce your highest level first, so you may lose levels in your original class if drained soon after the switch. If your levels are tied, the original class gets drained first.
Rodrick the Cleric adventures until he's 3rd level. When he finally earns 6000+ XP, he can choose to become a 4th level Cleric, or he could dual class.
If he dual classes into Fighter, he has 2 1st and 1 2nd level cleric spell and can Turn Undead up to wraiths. He can also now use any weapon and gains 1d10+Con modifier hit points (yeah, I use AD&D hit die types). He uses Cleric saves for all categories but Dragon Breath, which is slightly better as a 1st level Fighter (in BX/BECMI, anyway). He gains the Dodge ability of Fighters (once per day, reduce damage from a melee or ranged weapon attack to 0). From now on, he can only progress as a Fighter.
I think this allows for a bit of flavor, won't be overpowering (you can't get 4th level spells then decide to be a fighter, for example, or be starting your new career as a 1st level magic-user with 58 hit points), and a lot less bookkeeping than AD&D dual classing or most rules for multiclassing. Also, at high level, the dual classer won't be so far behind others in their active class.