As I've discussed before, I'm combining the Fighter and Thief into a single class in my Super-Casual Pulp Campaign - for players, that is, but not necessarily for NPCs. Here's how it'll work.
Fighter/Thief - A class for Player Characters in Basic D&D and similar games
Prime Requisite: Strength or Dexterity, whichever is higher. (If Strength is higher, apply XP Bonus as Fighter. If Dexterity is higher, apply XP Bonus as Thief. If both ability scores are equal, it doesn't matter which one you use.)
Hit Dice: As Fighter
Attacks: As Fighter
Saving Throws: As Fighter
XP per Level: As Fighter
Maximum Level: 36th
Armor and Shields: Any, but cannot use most Thief abilities if wielding a shield or wearing armor other than Leather
Special Abilities: As Fighter, and as Thief (with some restrictions: see below)
Race: Humans only
Thief Abilities: This class can Open Locks, Find and Remove Traps, Pick Pockets, Move Silently, Hide in Shadows, Climb Sheer Surfaces, Hear Noises, Backstab, Read Languages, and Cast Spells From Magic-User Scrolls as a Thief of the same level. However, they can only use these abilities if they are not using a shield or wearing armor other than Leather, with the exception of Reading Languages.
At the Dungeon Master's discretion, this class may also be forbidden from using these same abilities if they are encumbered by more than 800 coins' weight.
Name Level: When a Fighter/Thief reaches 9th level, they must choose between either building a Castle/Stronghold and becoming a Baron/Baroness as per the Fighter class, or constructing a Hideout and attracting Thief followers as per the Thief class.
If your campaign includes other alternative Name Level options for Fighters and/or Thieves, such as becoming a Traveling Fighter, Paladin, Knight, Avenger, or Travelling Thief/Rogue, these are also allowed, but all of these options are mutually exclusive.
This does not alter the abilities of the Fighter/Thief described above, but it does limit the territory, construction projects, and attracted followers of the character to those of either a Fighter or Thief of Name Level.
Restriction to Player Characters: NPCs cannot be members of this hybrid Fighter/Thief class. NPCs of the Fighter or Thief classes function as usual, including retainers, hirelings, mercenaries, and henchmen. However, if a Non-Player Character becomes a Player Character, as in the case of a PC dying and one of their retainers becoming the new PC for the same player, the Dungeon Master may allow the character to be "promoted" to the Fighter/Thief class. (In fact, the DM is encouraged by this humble blogger to do so.)
Alternatively, the DM may allow retainers to be members of the Fighter/Thief class (since they are at least partially controlled by players) while excluding other types of NPCs.
Thieves who are promoted to the Fighter/Thief hybrid class are converted to the level indicated by their current amount of XP and the Fighter XP table, rather than their old level as a Thief.