I always liked milticlassing. Most of the time I liked having some other abilities to supplement my main class. It could be a bit of magic to back up thieving or some healing to back up the fighting, didn't matter. Early editions I accepted the level of power for a character that was level 5/5 was not equal to a single class level 10, maybe equaled a level 7 or 8 power level. That is the trade off.
I liked 3e where each class was more ballanced and each race and class could multiclass around rather at free random. It was broke down at the lower levels and taking only 2-3 levels of some classes happened a lot. It did allow me to make any type of class idea I had come to life.
Somewhere along the 3e into 4e line more classes kept coming out with each new book. Now I do not really need to take a few levels of one class and add to the few levels of another to get what I'm looking for. I can just make a new class of levels 1-30 and be done. If I want a thief/ wizard I can, but not to get confussed with my wizard/ thief whom has several other abilities and is more a wizard than thief. I just name them different. I can do away with paragon paths and prestige classes this way as well.
I still like the idea of milticlassing, but see the need for it being less than from the time when we only had like 6 classes.
The other idea which would take more up front work is to take each class and split it into a major/ minor part. Your class you take at first level, the class you spent years of your childhood honing to become, would be your major class. you get all the cool powers for the class. You get the spells and the familiar, you get all the armor and weapons, etc. Your minor class is what you decide you want to start learing because the cleric died and you think your party needs more healing, or you want to add some wizard abilities to your thief. When you MC you only get the minor parts. I do not know how to slice it to make it still cool to add another class, but a bit of a penalty for doing so. This may be ballance out by keeping some of the power the same, like having character level and not class level for some things.