Not that I think they are going to do this (for a variety of reasons and intuitions), but this kind of theme would really smooth out the rough edges in one of my ideas for a fixed set of classes. One of the problems that never seems to quite get addressed with class/multi-class is certain intersections that aren't truly represented well by multiclass. For example, if you have fighter as a good single class and wizard as a good single class, then fighter/wizard seems to either be over-powered, radically gimped, or dependent upon something that you'd really rather not be--i.e. being an elf in Basic. Wizard/Cleric is even worse.
So my thought on that was to make the core classes as:
Fighter (martial), Wizard (arcane), Cleric (divine), Rogue (skirmish)--plus these synergistic, complete mixes (pick stuff from each side that works well as a single package, names could obviously use some work):
- Paladin - martial/divine
- Gish * (Need a better, but still one word name) - martial/arcane
- Ranger - martial/skirmish
- Priest - arcane/divine
- Bard - arcane/skirmish
- Druid - divine/skirmish
Now, multiclassing between, say, Cleric/Paladin or Cleric/Priest gives a very definite slant to the character, but probably doesn't run into any kind of stacking over/under powered issues, because those are nuances on what is already there.
Then you handle things like shapeshifting, bardic music, etc. either as what makes those particular classes stand out or push off to another part of the system (feats, spells, etc). The main problem with
that solution is that some of those things don't really fit anywhere. So you end up shoehorning them into the class. Now all your divine skirmishers turn into bears and all your arcane skirmishers play music.
Themes nicely solves that problem by providing another place to hang that stuff, while still limited it to a niche. That is, one of the best limits of the class is the opportunity cost--pick class X, you don't get A, B, and C. Themes provide a separate silo that thus doesn't impose this opportunity cost on your class selection(s), but does impose it on your potential other theme selection(s).