Pick one of these at first level. You can either double down on your archetype (such as a magic-user taking an extra spell slot) or grab something from another class (like a fighter with climb walls). Or you can roll 1d20 and trust in the Norns. This should allow the creation of many different hybrid character types with minimal fuss. I was working with the Basic rulebook in mind, but I bet this would work with many other, similar systems.

1. Magic-user spell slot (and one spell)
2. Scroll casting (and one scroll to start with, determined randomly)
3. Wand proficiency (and one wand to start with, determined randomly)
4. Cleric spell slot (and one spell)
5. Turn (or command) undead as cleric of equivalent level †
6. Tough/lucky: +2 all saving throws ‡
7. Increase hit die one step (for example, d4 becomes d6)
8. Dwarf-sense: notice dungeon features as dwarf
9. Elf-sight: infravision and find secret doors as elf
10. Harrier: +1 missile attacks, AC bonus of 2 vs. large creatures (as halfling)
11. Backstab as thief
12. Pick pockets as thief of equivalent level
13. Climb as thief of equivalent level
14. Move silently as thief of equivalent level
15. Hide in shadows as thief of equivalent level
16. Open locks as thief of equivalent level
17. Remove small traps as thief of equivalent level
18. Hear noises as thief of equivalent level
19. Armor proficiency (ability to use the next better category of armor)
20. Weapon proficiency (ability to use a weapon not usually allowed)

My Basic Rulebook

Some of these options are likely to be redundant (such as a thief with “open locks as thief of equivalent level”). You can either not pick such options, re-roll them if rolling, or use the following guidelines for handling redundant options. Weapon proficiency becomes weapon specialization, and means +1 to attack and damage with the weapon in question. Armor proficiency becomes armor specialization, and adds a bonus of 1 to AC when wearing armor. A redundant thief skill grants a bonus (+1 if using d6, +15% if using d%). Redundant backstab adds an extra die of damage. Redundant elf-sight or dwarf-sense adds +1 to the d6 roll. The “harrier” feat stacks with the halfling class. Redundant turn undead grants a +1 to the 2d6 roll. Redundant scroll caster or wand proficiency? I don’t know; make something up. Maybe it’s just a free item at first level, or extra charges when using wands, or bumping up the exhaustion die if using something like abstract wand ammo.

You’ll note that there are not really any new abilities on this list. That is intentional, to keep the options as familiar as possible. If you’ve ever played Basic D&D, it should be super obvious how all of these things work.

† Choose either turn or command when the feat is taken.
‡ +2 is equivalent to the difference between the dwarf and fighter saves in the Basic rulebook.