Example: You want to play a Fighter, but would like the benefit of being able to heal small amounts of hit points. You could start as a Leech for a couple levels and switch to Fighter later, but another option would be to take just the Leech healing ability as an add-on to the standard Fighter class. You would be a Fighter+Healing and would have to earn more XP to increase in level.
I hadn't settled on a way to handle it, but now I'm thinking my original suggestion for experience works well: The cost for one class ability is half the normal experience for that class, and there is a 6th level cap on that ability. So, a Fighter+Healing needs
- 2,600 xp to reach 2nd level,
- 5,200 xp to reach 3rd,
- 10,400 xp to reach 4th,
- 20,800 xp to reach 5th,
- 41,600 xp to reach 6th,
This assumes that Talents -- skilled classes like the Thief, Leech, and Apothecary -- have three abilities that are skill bundles. For example, the Thief abilities are Surprise (backstab and hear noise,) Stealth (Move Silent, Hide in Shadows, Pick Pockets) and Cunning (Open Locks, Disarm Traps, Read Languages.) A Fighter could be a Fighter+Surprise, Fighter+Stealth, or Fighter+Cunning variety, taking just one of the three abilities.
I'm debating whether to allow a single spell to be taken as an add-on. The character would not be able to memorize other spells or make scrolls, and probably should be restricted to spell levels 1 through 3, but would be able to cast a single spell 1 to 4 times per day. The cost would be trickier, though; it would start at half the experience of an M-U able to cast 1 spell of that level. So, a Fighter+Charm Person would need 3,250 xp to reach 2nd level, but a Fighter+Fireball would need 12,000 xp to reach 2nd level. That's not very elegant, hence my reluctance to commit to it yet.