Point of order: the intent of sidekicks is explicitly not as something for less experienced players to play, but as a way to give a small party (particularly single-players) a bit of a boost in utility, and another character to interact with. You certainly can hand a newbie a sidekick, but that’s not what they were designed for. The reason for making Sidekicks instead of monsters/NPCs with class levels is so that a solo player can more easily manage controlling one in addition to their full PC, or the DM can more easily manage them in addition to the enemy monsters/NPCs.
Now, since those of us who haven’t gotten early copies haven’t seen the final Sidekick rules, I am taking it as a given that Sidekicks will be simple enough for a even a new player to reasonably control alongside their character, since that is the explicit design intent. Obviously if they fail to live up to that intent, Sidekicks as class features won’t be a viable option. But for the sake of this discussion I’m going to assume they do live up to their intended function until the box comes out and proves otherwise.
So, with the knowledge that Sidekicks are meant to supplement small parties and the assumption that they will be reasonably easy to manage alongside a full PC, I’m interested in the possibility of using them as class features. Potentially replacing Extra Attack for martial characters, though that was just my first idea, I’m open to other possibilities.