Jackson Malloy at Sword and Scoundrel has blogged about a houserule for hybrid classes in Lamentations of the Flame Princess here: https://swordandscoundrel.blogspot.com/2015/07/lotfp-multiclassinghybrids.html
The work here expands on those ideas, with some additions, modifications, and clarifications. If you don't already have the free rules for LotFP, grab them at RPGNow, DriveThruRPG or DMsGuild.
In short, under Malloy's system, you can combine any two of the four human classes into a hybrid class, with the exception that combining two spellcasting classes is not allowed. Picking Fighter gives you +2 attack bonus (non-increasing) and their combat options (Defensive Fighting, Press, Parry +4). Picking Specialist gives you half the amount of skill points, and you are locked to two skills you pick at the start. Picking Magic-User or Cleric gives you spellcasting as if you were a spellcaster of half your character level. Saving throws are as either one of the two classes (player chooses at the start).
While there have been some attempts to add Paladins and the like to B/X games, such as the holy character in the Basic Fantasy RPG Quasi-Class Supplement, I really like the simplicity of Malloy's system. It's the minimal mechanical change you can make to add classics like Paladins to LotFP (an extremely focused and precisely-crafted game). There's no Lay on Hands bolted-on here - a Paladin is literally a nerfed Fighter with divine spells, and a Bard (if that's what you decide to call it) is a skillmonkey with arcane spells.
However, Malloy’s system is not quite complete - glaringly, nothing is said about hit dice/hit points, or about spellcasting encumbrance limits. More on that later, but first, let’s map out the hybrids available. (From hereon, I’ll be abbreviating Fighter, Specialist, Cleric and Magic-User as FTR, SPE, CLE, and MU for my own sanity.) The five combinations are:
- FTR-SPE (“Professional”: Ranger, Assassin or similar)
- FTR-CLE (Paladin)
- FTR-MU (Spellblade, or whatever other nonsense term for a gish)
- CLE-SPE (Seeker)
- SPE-MU (Bard, inevitably).
Now, instead of introducing a "player-facing" mix-and-match system, let’s pre-mix the options and write them out into complete classes or archetypes. Because we want to limit the complexity we present to players, and want them thinking about minmaxing the system as little as possible. So instead of letting the player choose the saving throw tracks, I will make the decision on which is appropriate for each archetype. For example, a Ranger is a FTR-SPE hybrid with Bushcraft and Stealth skills and saves as a Fighter. Assassin has Stealth and Sneak Attack and saves as a Specialist. If you want, you can add more: a FTR-SPE with Search and Tinkering could be a Tomb Raider (saves as a Specialist). In case you’re wondering what a Seeker is, it’s what I’m calling what is essentially a grave-robber in the service of a church, who goes out to recover religious relics.
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Hit diceFor hit dice, I think the way to go is to split the difference and round up, so a hybrid of a d6 and d8 class would have a d8 hit die, and a MU combined with anything else would have a d6. Thus, though both are half-casters, the Paladin has more hit points than the Spellblade, which is what I would expect to see.
ExperienceThen there is the matter of experience. XP required to level up in Malloy’s system is equal to the higher of the two classes, plus 50%. For example, CLE-FTR: 2,000 x 1.5 = 3,000. FTR-MU: 2,250 x 1.5 = 3,375.
However, let’s examine what a Spellblade (FTR-MU hybrid) gets over the Elf (who requires only 3,000 XP). Elves cast practically as a MU of their level. Spellblades cast at only half their level. Since I’m giving them a d6 hit die, they don’t win over the Elf in hit points or saves either. Plus, Elves have other special characteristics. Though I think it’s fine for the Elf to be as good as it is, I still think we should aim for the best possible balance when adding these new options, so the Spellblade should not cost more experience to level than the Elf - in fact, it should cost less.
On the other end of the scale, the hybrid class should certainly cost more XP than either of its constituent classes. And I believe it should also cost more XP than the Fighter, even in the case of the CLE-SPE, just because. So, we have a range of XP to aim for with our base XP (experience required for level 2, which is what all subsequent levels are calculated from) formula: more than 2,000, but less than 3,000. This is the equation I came up with for the XP required for level 2:
Hybrid class base XP = (Class 1 base XP + Class 2 base XP) * 2/3
To keep the tables clean, I rounded the results to the nearest 100.
This results in the following XP requirements for level 2:
A nice spread, in my opinion.
Side note: In case you're wondering, the formula in B/X for calculating the experience required to gain level X is: Base XP * 2^(X-2), for X>1.
Casting spellsThe maximum encumbrance levels for spellcasting in LotFP goes like this: light - heavy - any. (MU - Elf - Cleric)
For our new classes, let’s say heavily encumbered is the default limit, and having FTR or CLE shifts it one level up, and MU one level down. The result: Paladins (FTR-CLE) and Seekers (CLE-SPE) can cast at any encumbrance, Spellblades (FTR-MU) at heavy, and Bards (SPE-MU) at light. The only strange result here is the Seeker, who usually wouldn’t be wearing armor, since they need to be unencumbered to use their movement skills. But for the Paladin, Spellblade and Bard, I think this conforms to expectations.
Cleric hybrids can prepare any spell, as normal. Magic-User hybrids learn Read Magic at 2nd level, when they gain spell slots. As for learning and researching spells, here's where you can add unique mechanics to make the flavour of world you want. Perhaps Bards don't write spells down, but record them in their head, and research spells by listening to songs and stories. Perhaps Spellblades don't write their spells into books, but etch them onto their weapons and armour.