Other: "concept."
Sometimes "concept" is heavily thematic, like "traditional Merlin archetype, with shapeshifting and subtle magic." Sometimes it's heavily optimization, like "(nearly) unhittable tank" or "main-healer paladin." Sometimes it's an axis that I feel gets overlooked far too often in these discussions, "procedural"--that is, thoroughly developing the most entertaining/engaging process of playing the character, even if it's not especially thematic nor especially optimal--e.g. "grapple and support."
Incidentally, all four examples are things I've actually tried to do in at least one TTRPG system, and three of the four were for actual campaigns even if I didn't end up using them (the thematic one was just a personal exercise, to see how close I could get to "Welsh" Merlin in 4e).
In the pursuit of a concept, I will weigh each of the three axes above (theme, effectiveness, procedure) until I find the right balance. No matter what the prime nature of the concept, I will (and have) made sacrifices in each area in order to reach that balance. Perhaps "subtle" magic doesn't mesh with "shapeshifting" in the rules; I may settle for "elemental" magic instead. Perhaps being a meaningfully engaging support character means you have to have spells (I'm looking at you, 5e), which is a procedural level I often find excessively intricate but will accept if it is the only path available. Perhaps the "optimal" path for a healer Paladin means dumping Intelligence, which I am loath to do because I don't like playing "dumb" characters. Regardless of the situation, I'll make sacrifices in any one area if the loss is sufficiently compensated in the others--and without careful thought and knowing the context, I can't rule out any particular change.