At best, it’s an opportunity to organically introduce other types of content and the associated software to people through the social graph they chose to participate in.
If you did as you suggested, by adding posts / threads / communities / magazines without the consent of the users, that would indeed be a problem.
Preventing Mastodon users from seeing the content after they made the choice is also a problem.
As for selecting an instance, if that is a barrier, then forcing users to create and use multiple accounts in different ways to see all the content they want to is an even larger hurdle to present to users.
One of the biggest problems with Mastodon is new users who think they need an account on every instance website they interact with.
@ThatOneKirbyMain2568