When I started roleplaying my first system was D&D 3.5 and a big part of that game was mixing and matching classes to get strange new powers. Thinking about Jared’s thread Fictional characters as Torchbearer 2e characters you can really expand the repertoire. I mean, how else are you supposed to make Durkon from Order of the Stick? 
When you level up you may choose to take levels of any class you meet the prerequisites for starting at 1. The prerequisites are the starting class skill ratings plus the trait for the non-human classes.
Burglar
Prerequisites:Cook 3, Criminal 3, Fighter 3, Hunter 2, Scavenger 2, Scout 2 and the Hidden Depths trait.
Weapon Benefits: Wield any weapon except Crossbow, Great Sword, Halberd and Polearm. Armour Benefits: Wear Leather, Helmet and Shield
Other Benefits: None
Magician
Prerequisites: Arcanist 4, Lore Master 3, Alchemist 2, Cartographer 2, Scholar 2
Weapon Benefits: Dagger and Staff
Armour Benefits: None
Other Benefits: Gain 1 Memory Palace, you may learn arcane spells.
Outcast
Prerequisites: Fighter 4, Dungeoneer 3, Armourer 2, Orator 2, Sapper 2, Scout 2 and the Born of Earth and Stone trait.
Weapon Benefits: Any weapon except Great Sword
Armour Benefits: You may wear any armour.
Other Benefits: +1 to camp event rolls in dungeons and dwarven-made structures.
Ranger
Prerequisites: Fighter 3, Pathfinder 3, Scout 3, Hunter 2, Lore master 2, Survivalist 2 and the First Born trait
Weapon Benefits: Sword, Spear, Bow and Dagger
Armour Benefits: Leather, Chain And Helmet
Other Benefits: +1 camp events rolls in the wilderness.
Theurge
Prerequisites: Fighter 3, Orator 3, Ritualist 3, Healer 2, Theologian 2
Weapon Benefits: Any one weapon except bow and crossbow.
Armour Benefits: Leather, Helmet and Shield.
Other Benefits: Gain 1 Urd, you may use relics.
Warrior
Prerequisites: Fighter 4, Hunter 3, Commander 2, Mentor 2, Rider 2
Weapon Benefits: You may wield any weapon
Armour Benefits: You may wear any armour
Other Benefits: You may keep watch and lead the effort to prevent a disaster for free provided you have no conditions (except fresh).
Shaman
Prerequisites: Ritualist 4, Theologian 3, Fighter 2, Healer 2, Scavenger 2
Weapon Benefits: Dagger, Hand Axe, Sling and staff
Armour Benefits: Shield
Other Benefits: Gain 1 Urd, you may use relics.
Skald
Prerequisites: Orator 4, Manipulator 3, Fighter 2, Lore Master 2, Scholar 2
Weapon Benefits: Battle Axe, Dagger, Hand Axe, Sling, Spear and Sword
Armour Benefits: Leather, Chain And Helmet
Other Benefits: +1 to the Telling Tales Events rolls in town.
Thief
Prerequisites: Criminal 3, Manipulator 3, Sapper 3, Scout 3, Fighter 2
Weapon Benefits: Bow, Crossbow, Dagger, Hand Axe, Mace, Staff and Sword
Armour Benefits: Leather
Other Benefits: None.