History repeats itself. The first time as tragedy, the second time as farce
Right. So here's how things used to work back in AD&D:
You chose a class, or maybe you chose several classes. Either way, you divided up our XP evenly into every class you had. Each class would give out the next level at a specific amount of XP, and that amount was a lot more than the amont the last level was handed out at.
The net result, of course, was that someone who had two classes, or even four, wouldn't be many levels behind someone who had only one. And when you got to 10th level in two clases and your friend was 12th level in just one, you weren't a 20th level character, you were a 10th level character. And this was maintained by the fact that every aspect of your character (attack bonus, saves, whatever) didn't add between classes. Each class had its own damn chart, and you looked up what value that class gave for each level - and if you had more than one class you used the best value out of the classes you had.
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And what's so very important about that? Well, it's exactly the fvcking same as the diminishing returns systems some people are jabbering about now. And it can be made to work. And it is neither new nor innovative.
What's really interesting about it, however, is the twin pulls on it: a multiclass character ends up being worse at his best trick, but has other tricks that he has at all (which are themselves worse than if those had been his best trick instead). So you really have a couple of problems generated: first of all, you've got to make sure that someone who multiclasses is getting an ability that matters; and of course you have to make sure that the cost matters as well.
So you can make that work. You just adjust the level point in which having more things in your toolbox is worth having less things in your toolbox. That's entirely plausible. But that's not compatible with the modern idea of free multiclassing.
Why not? Because in free multiclasisng, even if you institute a scaling specialization cost (either in XP or in actual levels, it matters not at all) you don't know how much strength they are giving up for diversity. That is, since people aren't necessarily splitting things evenly, you can't know in advance what ratios of levels people will actually have.
So you can't just say "having magic and swords is worth being -3 in each" - because people won't necessarily do that. People will both be trying to game the system and trying to express weird character concepts that your system doesn't support very well.
The problem is inherently intractible as soon as you don't divide up the XP evenly, because the costs of diversification are no longer fixed. Therefore it becomes impossible to make sure that none of the PCs get power for nothing or spend power on new abilties that they can't actually use.
Not difficult, impossible. Whatever you set the mystic theurge point to, people will nonetheless take more and/or less levels of the second class and end up outside your balance point.
-Username17