For those who don't know: Mastodon works differently in how you build your social graph than Twitter, where much is automated by the recommendation algorithms kicking in. I think that once one properly grasps how to go from 'empty timelines' to having some first fine people in 'following' the process of building further can very intuitive and be a refreshing and liberating experience where one feels in control.
As for ideas and feedback..
"follow suggestions [based on] most popular/proactive accounts in your language". I would be careful here, to not get to a point where Fediverse / Mastodon becomes dominated by influencers that are focal points of activity and sees this feature becoming part of their SEO marketing toolkit. While it may help uptake and network effects, it may hurt and degrade fedi culture over time.
You might consider integrating something similar to Trunk [0] so people can directly follow others that have explicitly expressed an interest in a particular topic and are open for discovery. They can peruse the timeline of these accounts and then checkmark the ones to follow based on that. This keeps social graph building natural and organic, not algorithm-driven.
As for "trending hashtags", another algorithm driven engagement feature. There may be a Hashtag Directory to browse, sort and filter and an indicator of post counts to each of them. Long inactive hashtags need not be shown, but the directory shows way more entries than just what's trending and doesn't directly push you to that.
Another idea might be to have account profiles be able to have a list of the top 3 or top 5 other accounts most valued by a person, in a friend-of-a-friend kind of way. Maybe it only becomes available once a mutual follow relationship is established, and allows further discovery of the FOAF graph.
