This is something I have been swatting around in my head for a while, and I wanted to see what you, the illustrious intelligent people of EN, thought of it.
I got this idea from the AD&D multi-classing system. Basically, when you make your character, you make a choice: you can play a character the normal way, one class at a time, multi-classing, taking prestige classes, etc. Or, you can make a gestalt character, but forgo ever multi-classing or taking a prestige class of any kind. Each player would decide individually, so gestalts would be in next to normals.
Obviously, the gestalt characters would be a little more powerful at the lower levels, so maybe there should be some system of XP penalties for the gestalts? Would it even out in the long run?
The only way I could see doing this would be to charge the Gestalt characters more XP to advance.
For instance, take a fighter(1)//cleric(1)... This character wouldn't advance until his/her total XP was enough to advance to 3rd level (using whatever XP chart you're using).
Using the 3.x XP chart, the character would need 3,000 to get second level in both sides of the gestalt level. Then, 10,000 XP to get to 3rd in both sides, etc...
That's how I'd do it, IF I did it this way.
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Question... AD&D had separate and different XP charts for each class. So, multiclassed characters would advance at different rates in each class. Any thoughts on how to mimic this in 3.X?
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My own houserule is that everyone that multiclasses advances as if they were semi-gestalt. Characters only advance in one class at a time, but when they multiclass, they only get the unique abilities that a level in the new class grants (kinda like Gestalt). For instance; BAB, Saves, skill points, do not stack... The character simply gets the best of what each class offers. However, I let hit points from all classes stack (as there's something special about multiclassed characters, in my view).
I didn't even used to let characters gain separate pools of spell slots from different classes. They simply get the best set available to them according to their classes and level in each class. However, I've removed that portion of the houserule to make multiclassed spellcasters more viable (and more powerful).
Off topic... I should mention that in my game, 6th level is considered unthinkably high level, but I don't cap progression there. XP awards simply slow down more drastically than standarad d20 or PF RPG. Each level requires about 7 more CR appropriate encounters than the previous level to award enough XP to advance to the next level. So, while 2nd to third requires 13.333 encounters, 3rd to 4th requires about 20. 4th to 5th... About 27... Etc... This is important in the issue of multiclassing, as a character that multiclasses will not keep up with single-classed characters if they all survive the same adventures together. This is because they won't have the same level of BAB, saves, etc... that a single-classed character would have. But, what they lack, hopefully they make up for with versatility. A single-classed character that makes it to unthinkably high level might wish they'd multiclassed earlier on as they see that they lack that versatility (albeit, too late).