I myself do not find character building much entertaining, but mostly because it is cumbersome and it is removed from the game world (i.e. my choices as an adventurer have no impact on which abilities I might be able to take). On the other hand, it feels rewarding to being able to pick a new ability and using it as soon as the right circumstances arise.
The solution might be making almost every character ability diegetic regarding their acquirement. That is, classes could be designed with only a few key differences (permitted weapon and armour; advancement of to-hit bonuses; hit dice type; and one or two distinctive feature), and we could provide ways of gaining new abilities tied to the setting. To keep niche protection, abilities could also be tied to certain classes (that is, a character may not acquire every available ability). Also, if abilities are not equal in power, the difficulty of their acquirement can be adjusted properly.