We all are aware of the problems of 3E multiclassing. I was thinking back to 2E, which seemed to give you a more genuine trade off.
In 2E you'd pick two classes and start out as level 1 in each. For things like Hit Dice, you'd roll HP and divide by 2 for each class. Back then, different classes advanced at different rates, and you had to track XP separately, but that wouldn't strictly be necessary for the concept to work. Because of the XP progression tables of 2E, it took double the XP for each subsequent level. Because your XP was being split between two classes, this effectively meant you were about a level behind everyone else. So, if the group was level 5, your Fighter/Cleric would probably be around 4/4.
That honestly didn't seem too bad. I'm not saying it's perfect, but you'd be advancing both classes at the same time, rather than one level at a time in 3E. The closest 3E got to this was classes like the Mystic Theruge, which was too little, too late. But, what if I took that basis and tried to make a PrC where you could stay just one level behind. This is a bit of a working prototype:
(Note: I was thinking this out with vanilla 3.5 rules, not Tome classes.)
Multi-class (Or, insert better name, here)
Prerequisites: At least one level in two different base classes.
Hit Die: An "average" of the two base classes. Add the number of faces on each die, divide by two, and round down to the nearest die type. So, a d10 and d4 = 10 + 4 = 14, divide by 2 = 7, round down to d6.
Base Attack Bonus: An average of the two base classes. I could write up a 7/8 and 5/8 progression table if needed, or we could round down to the existing three 1/1, 3/4, and 1/2 progressions. Personally, I'd prefer the first approach.
Saving Throws: Each saving throw is an average of the two base classes. Again, we could come up with an intermediate progression that is an average of the good and poor progressions. *
Skill Points: Get an average of the two classes. All skills from both classes are class skills.
Weapon/Armor Proficiencies: Nothing new gained.
Class Features: Each level, you gain all the class features of both base classes as though you'd gained a level in each.
* Also, I know every base class and PrC gives out that +2 bonus to every good save at level 1. If we wanted to pretend that this PrC wasn't actually a separate class, we could forego that bonus. I'm not heavily set on either approach.
So, how well would this work in practice? It's simple, and relatively easy to implement. I'm curious how abusable this is. The obvious benchmark is casters. They're what make the world go round. Doing this would cost you a caster level, which means you'd be behind in your highest level of spell available half the time, and half the time you wouldn't be.
The direct comparison would be the Mystic Theruge. At level 12, you'd be a Wizard 11/Cleric 11, which is pretty damn solid. That's a lot of spell slots, although, a Wizard 12 or Cleric 12 has quite a few by that point, too. Compared to the Mystic Theruge at level 12 would be casting at 9/9. That's clearly too weak, but is 11/11 too much? Probably your biggest gain would be at levels 3 through maybe 5 or 7. You'd get a lot of extra spell slots relative to what you'd normally have at that level straight-classed.
Similarly, you could take some martial class to beef up your chassis a bit at the cost of a caster level. You're probably better off just straight-classing.
Classes like Rogue could be thrown in for lots of Sneak Attack progression. I'm sure you could figure out ways to cheese it and get some pretty good damage progressions with touch spells. Still, I'm not sure it's that much worse than a straight caster.
Regarding martial classes, you could throw Rogue in and it'd be almost a straight-up improvement, at a slight cost to HP and BAB. I don't see this as a problem, as those classes get worse and worst as you gain levels, anyway.
Is there anything I'm missing? Obviously, some combinations fare better than others. Does this break the game terribly, or is it still the same Mystic Theruge problem, but just less so? I'd made something like this a while ago that required more sacrifice of class levels, but I wanted to try this 1/1 entry to see if mimicking a 2E multiclassing system was viable.