I hate the idea of multiple XP tables. Milestone XP streamlines the game too much. Frankly, I want Proficiency Bonus and Hit Dice to remain consistent across my players. Similarly, I don't want any irritating rules where you can't combine abilities or have to track things separately. I
like the combined spellcasting chart in the 5e multiclassing chapter.
Here's how I'd imagine Gestalt/hybrid/AD&D multiclassing. The drawback to this method, as always, is complexity.
Select two or three classes. Then:
1. Remove all path options from the classes. Mutliclassing sacrifices the benefits of specialization, so multiclass characters never select a class path for any of their classes.
2. Skill proficiencies: Select two of your classes. Select two skills from either of the skill lists for the selected classes. If one or more of the selected classes is Bard, Ranger, or Rogue, instead select three skills from either of the skill lists for the selected classes.
3. Save proficiencies: Select one of your classes. Gain the save proficiencies for that class.
4. Weapon, armor, tool proficiencies: Combine the weapon, armor, and tool proficiencies of all your classes.
You gain levels normally, and your level is used to determine your proficiency bonus and hit dice. However, your class ability level is reduced, so you will not gain new abilities each level.
5. Proficiency bonus: Your character level determines your proficiency bonus. A level 11 Fighter/Wizard has the proficiency bonus of a level 11 character.
6. Hit dice: Alternate hit dice each level, so a Fighter/Wizard would use d10, then d6, then d10, then d6, etc. A Fighter/Rogue/Wizard would use d10 -> d8 -> d6 -> d10 -> d8 -> d6, etc. You must always keep your hit dice within 1 of each other. A level 13 Fighter/Rogue/Wizard would have 5d10 + 4d8 + 4d6 Hit Dice. You would not be able to gain a sixth d10 hit die until you had both five d8 and five d6 hit dice.
7. Use the chart below to determine your character's class ability levels. Your character has the abilities of the class ability level. Thus, a level 5 Fighter/Wizard has a class ability level of 3, and has the class abilities of a 3rd level Fighter, and the class abilities of a 3rd level Wizard. However, such a character would have a +3 proficiency bonus that a 5th level character has, and would have 3d10 + 2d6 Hit Dice from levels. Again, you never select any class paths, so you never gain any abilities tied to class paths.
Code:
+-----------------+--------------------------+----------------------------+
| Character Level | Dual Class Ability Level | Triple Class Ability Level |
+-----------------+--------------------------+----------------------------+
| 1 | 1 | 1 |
| 2 | 1 | 1 |
| 3 | 2 | 1 |
| 4 | 3 | 2 |
| 5 | 3 | 3 |
| 6 | 4* | 3 |
| 7 | 5 | 4* |
| 8 | 6 | 4 |
| 9 | 6 | 4 |
| 10 | 7 | 5 |
| 11 | 8* | 6 |
| 12 | 9 | 6 |
| 13 | 9 | 7 |
| 14 | 10 | 7 |
| 15 | 11 | 8* |
| 16 | 12* | 9 |
| 17 | 12 | 10 |
| 18 | 13 | 10 |
| 19 | 14 | 11 |
| 20 | 15 | 12* |
+-----------------+--------------------------+----------------------------+
* Indicates a level at which the character gains an ASI.
For the curious, the formula for class ability level for 2 classes is character level * 0.75 rounded down. For 3 classes, it's class ability level = character level * 0.60 rounded down. Minimum level is 1, obviously.
8. When a character gains an ASI at class levels 4, 8 and 12, the character gains only
one ASI, not an ASI from each class. Similarly, abilities described in the multiclass section as overlapping, such as Extra Attack, still overlap.
9. For spellcasting slots, use your character level and the combined spellcasting chart in the multiclassing chapter to determine your slots. If one of your classes is Bard, Cleric, Druid, Sorcerer, or Wizard, then your character slots are determined by your character level. If none of your classes is Bard, Cleric, Druid, Sorcerer, or Wizard and one of your classes is Paladin or Ranger, you have spell slots equal to half your character level. For example, a level 10 Ranger/Rogue has class abilities as a 7th level Ranger and as a 7th level Rogue, and has spell slots as a 5th level character on the combined chart. For another example, a level 11 Fighter/Wizard has a class ability level of 8. You would use the level 11 on the spellcasting chart to determine the number of spell slots per day, but would prepare spells as an 8th level Wizard.
10. Cantrips, which normally key to your character level for scaling, instead use the class ability level. So a level 20 Fighter/Wizard casts
fire bolt as though her total character level were 15, not 20. Other abilities may also use the class ability level instead of the character level.
At this point, I would do some playtesting. Where things are at above is what I think would be slightly overpowered. My guess is that there's some discrepancy in balance between the two class and three class level charts. I'd step down the factor 0.05 each time. The balanced factor might be closer to 0.15 lower than it is above, so two class might need to be down to 0.60 (level 20 = 12), and three class might need to go down as far as 0.45 (level 20 = 9).
You'd have to make some special rules, I'm sure. Warlocks and clerics are a bit strange without their paths. Certain ability combinations are really good, like Fighter/Paladin character level 15, or Fighter/Rogue/Paladin at character level 19. My guess is that there are some combinations that are simply too good, so you might need to limit the classes or class combinations in some way.
The skill, save, and proficiency rules could be made a whole lot more complex to add flexibility, but I don't really feel like that's worthwhile.
I love 1e/2e multi-classing. Here is a website someone posted a while back with some 1e/2e style rules for 5e. I cannot remember who linked it...
