After seeing yet another discussion about classes advancing and specializing as they do, I’m moved to spill some thoughts on the matter. It’s something I’ve thought about often as I’ve been designing, with the usual to and fro in feelings as I consider different aspects.
I’ve landed on the belief that such an approach is laudable…with some caveats. The foremost caveat is that the advancement needs to be believable in diagetic terms. I expect a reasonable simulation of the setting in all things, and this is no exception. Your fighter can specialize into a sub-class, yes, as long as it’s believable.
Looking at AD&D, for example, where the abilities of Rangers and Paladins can be far removed from the abilities of a basic Fighter. Alongside the expectation that, in the setting, each PC had to spend time as an apprentice or trainee to gain class abilities, I’d expect that gaining all of the special abilities of a Ranger or Paladin would require much time spent training. The character isn’t going to wake up one day and suddenly be able to lay on hands. The player can have the PC retire from play for a while to reflect becoming an apprentice training in the new abilities; it isn’t going to simply happen with an increase in level on a random Monday morning.
The sort of specialization that does make sense involves adding a bit of emphasis on an existing ability or two, and then adding new abilities over time after that specialization. Gygax’s Thief-Acrobat was an early riff on the approach, with a freeze on development of some abilities and the addition of others; failing in adding emphasis to any existing abilities (IIRC).
I also think that such specializations should vary by what level the PC is when making the switch. An 8th-lvl Fighter becoming a Ranger, in addition to requiring a great deal of time for training, shouldn’t reappear in play as a 8th-lvl Ranger, complete with spellcasting ability. The PC just completed “Ranger Basic Training” without any real time spent in the field and can now cast spells just because? When it comes to level-based abilities like that, I expect the now-specialized PC to have to wait a bit. Sure, the HD and attacks stay the same as an 8th-lvl Fighter; any lvl-based abilities as a Ranger take extra time to appear.
If a designer truly wants to allow for sub-class specialization, I reckon it’ll be cooked into the system from the outset. I’d rather see sub-class options unlocked at an early level (have to be chosen upon reaching 3rd or 4th lvl, for instance), than an ad hoc approach for sub-classing at any level. E.g., All fighter begin as fighters and then can sub-class any time up to reaching 3rd level without ever having the option to do so later. (This is tied to my thoughts on using character classes, in general.)
The choosing of a sub-class should be well-regulated, in other words. The possibility should be worked out well in advance to fit in with the general simulation of setting. Sub-classing after character generation can be a good thing if planned for in the design at the outset. A fighter could begin as a trooper (mounted specialist) or begin as a foot soldier and then specialize later, with suitable time and adjustments to herald the change.
(As you can likely guess, I’ve never been a fan of changing classes in D&D.I shudder when I hear of players “dipping” into a class for a level in tales of 3e and beyond. That sort of thing throws me completely out of the setting and game at hand.)