Technically speaking... isn't the DMG the first splat book?
If you want to get technical, isn't the PHB the first splat book?
I mean, it's basically a big book of extra race and class and character building options + a physical reference for the rules.
Ruin Explorer said:
In 4E, they had the Hybrid system, which was conceptually somewhat similar, but because of the way 4E worked, it was actually possible to make it balanced, and not overpowered, and it generally worked very well.
Well, most of them were more likely to be UNDER-powered, so for certain values of "balance" I guess.
My main gripes with the hybrid system was (a) every class in 4e had to have some special snowflake artisinal hybrid version you could take, or else you it would not blend, and (b) there was weird silo-ing, so that, for isntance, if you had a striker bonus die, it would only apply on your striker-class attacks, so that it was more like you were either one class OR the other, and could swap back and forth, than like you were a true hybrid of both, greater than the sum of your parts.
Gestalt doesn't have a history of worrying about wrapping our games in bubble-wrap to protect their precious little maths. In 3e, it was just straight up awesome (and was "balanced" by just raising the encounter level!), and the old 1e/2e multiclassing it was meant to emulate had a kind of long-term balance.
I'd take the 3e system with some more concrete guidelines on encounter balance with a side of fries and be happy, honestly.