Of course, since having an additional character would effectively multiply a player’s action economy, I thought it might be best for Sidekicks to replace Extra Attacks if they were to become class features. And maybe replace cantrip scaling for casters?
What about characters who gain no extra attacks nor cantrips? Also, if all my cantrips don't scale, do I get my sidekick for free? The main problem here is that extra attacks and cantrips are not comparable with each other, and they are not comparable with sidekicks either.
In 3e there was a core feat, although in the DMG, called "Leadership", which granted you a cohort (plus various followers) of a lower level. If a DM wanted to give sidekicks as a character option (not mandatory), my first idea would be to consider it cost a feat, because at least a feat has the same cost for everyone (even if two classes get 1-2 more feats than others, in practical terms it matters little). Still I couldn't say if a sidekick is worth less or more than a feat...
I don't even know what is the final version of sidekicks in the upcoming book, but in general I think it would be best not to try and see as something that is part of the character: if you do that, immediately the game is seen unfair if some player get a sidekick for their PC and others do not. So then everyone will want a sidekick, and the DM has double the amount of characters in the game.
The
real point of sidekicks is explicitly to help a party with too few PCs (in the Essentials it seems it's even suggested only for "solo" games), without the complexity of a whole second PC for the player.
Instead, I have simply allowed experienced players to play 2 PCs at the table, when the number of players is small. It's not a privilege for such player, it's a burden to manage two whole characters, each one takes a single share of treasure and XP. Sidekicks can help lessen the burden, especially if the sidekick is chosen to be a spellcaster in order to fill a gap in the party's capability.
Since in both cases the motivation is to increase the
party, there is no need to have the player pay a price by reducing the first PC's features.
Make sidekicks a character option, and competitive players will look at it as an opportunity to be better than other players, if the "price" is right. Kind of a self-fulfilling prophecy. The problem is that it's very hard to set the price right. As I said, I would start with something that at least is shared among all classes: feats, proficiency bonus or even XP. Thinking of sidekicks in terms of action economy is interesting, but problematic once you consider that some sidekicks may be useful only in combat while others may be designed to be maximally useful outside of it.