Ask HN: What do you miss in language learning apps?

hi everybody! i feel like duolingo, along with any other language learning app, is simply not useful for serious progress. thats why im working on a personal project (will probably open source it at some point), that will be a lot more strict, grammar focused and will have a clearer roadmap - not just random exrcises (which i often feel is the case with duolingo). i would love to hear what you miss when using duolingo or other simillar apps.