I never understood the idea that a player should be rewarded for staying single-classed. The multiclassing system in 3ed was supposed to allow for an infinite number of combinations so a player could create the character concept they wanted.
Sure, there may be some combinations that are more attractive than others but I've never seen any combo that "breaks" the game. And trust me, I've done my share of trying.
Multiclassed spellcasters do need to be fixed, but any "fix" should be in accord with pro-multiclassing.
I totally agree with this. I have always felt the classes where just a package of abilities. Contrary to you, I have seen characters that do break the game.
I really like the 3e multi-classing system (about a billion times better than 4e) it gives an almost limitless number of possibilities. However, it does have several problems. Primarily saves, BAB and spell casting progression could use the fix. In addition to spell casting progression there are a lot of other progressions (familiars, animal companions, unarmed strike, sneak attack etc.) that could do with a general rule to keep them competitive as a character levels up.
Caster level and class feature progression fix
I personally like the rule that your caster level is equal to the spell casting class level plus 1/2 of all your other levels up to a max of double. This would also cover all of the scalable class features such as animal companions.
Saves fix
your saves are 1/2 your overall character level. Classes give a non-stackable +2 bonus in certain saves (Fighter FORT, Rogue REF, Wizard WILL, Cleric FORT and WILL etc.)
It might be nice to give everyone a +3 total rather than have some with a +4 and others with a +2 and only a few with +6.
BAB fix
Full gets BAB +4/level
3/4 gets BAB +3/level
1/2 gets BAB +2/level
Add all your BABs up and divide by 4. This is simpler than adding fractions.
Put it all together
So, if you had a fighter 2/cleric 8/wizard 4
BAB +10 ((8+24+8)/4=10)
Base FORT would be +9 (14/2=7 fighter gives +2 cleric gives +2 -> don't stack)
Base REF would be +7 (14/2=7)
Base WILL would be +9 (14/2=7 cleric gives +2, wizard gives +2 -> don't stack)
Wizard Caster level would be 8 (4+(10/2)=9 but can only get a max of double original level so 8)
Cleric Caster level would be 11 (8+(6/2)=11)
Anyway which class features which scale by half your other class levels would have to be written into the class features themselves. Some attempt at dealing with this would make the game better as a whole.