For my next campaign I wanted something like AD&D style subclasses with less of the hassle. So I'm calling the result Bonus Classes. A Bonus Class is a very simple layer over an existing class, with an additional stat prerequisite. So unlike AD&D subclasses, you don't hassle with their own rules anywhere else. No custom spell lists or experience point charts, for example. So here's a first draft at a Bonus Class for each of the canonical seven BX classes. Note: no attempt has been made to balance this stuff.
Psi Lord (MU Bonus Class), Prerequisite: Wis 13+, you may use the psionic rules from pages 19-21 of the LotFP version of Geoffrey McKinney's Carcosa. Also, you have a big bald head.
Ninja (Thief Bonus Class), Prerequisite: Str 13+, you get access to the list of cool ninja gear (forthcoming) and whenever attacking from surprise you get d4 additional attack rolls
Adept (Cleric Bonus Class), Prerequisite: Int 13+, each level starting at first you get one random MU spell added to your cleric spell list, this means you actually get a spell at first level
Malformed (Dwarf Bonus Class), Prerequisite: Cha 9-, you may roll d4 mutations/defects
Gray Elf (Elf Bonus Class), Prerequisite: Con 18+, each level you gain a random MU or Cleric spell (you choose which list to roll on) as a daily power