- I wrote about barriers to entry for subclasses and switching classes.
- I responded to part of JB's comment on switching classes.
- JB's posts on subclasses and variants: part 1 and part 2 and part 3.
I like limiting D&D to the four core classes, although I sort of redefine two of them. There's the fighting class, the magic class, the talented class (thief,) and the hybrid class (cleric.) I do see the value in having "other" classes, but I want them all to be variants of the four core classes.
The main way I make variants is by swapping one class ability for something that's roughly equivalent. The most obvious example of this is to change the way a spell-caster casts spells. I think of these as subclasses, although in a sense which class is the core class and which is a subclass of that class is arbitrary. For me, the main "class" is really just an XP/HD scheme and sample class abilities, and a subclass is any specific example using that XP/HD scheme.
My recent inverted classes all work on that same subclass principal, except that there's an additional limitation paired with an additional ability. The limitation is always "in world", not a system-level number manipulation. So, instead of a minimum ability score and an XP penalty, nobles and cavaliers have crazy social fears as a limitation.
There's another kind of variant I haven't quite worked out yet that's not what I would call a subclass. The usual term is "prestige class", but I don't like that term because (a) it suggests 3e, and (b) it implies that they are, well, prestigious, and maybe not all of them are? For that matter, maybe the shouldn't even be classes... I'm talking about a paladin's abilities, for example. I think "bonus abilities" is a good enough label. Some of them don't need any more than training rules. Others might have in-world restrictions. For example, my last attempt to fix paladins gave a handful of abilities to any character that enters a holy order and offloaded almost all of the other paladin abilities to holy swords. There's an XP "penalty", of sorts, but it's in-world: members of holy orders only earn experience from Chaotic monsters and the treasure thereof, and only if they also tithe.
I had some really old posts that rewrote monks as if they were bonus ability packages instead of a separate class, but these suffered from using ability score minimums. A later post gave a sketchy monk class based on the Delving Deeper thief, but I didn't write that up using my now-standard class template. I'm torn whether to go that route or the bonus ability route. Perhaps... both?