What anime is the biggest wasted potential?

I like Charlotte and think it’s a pretty good show, but man the last episode is so rushed. It starts a completely new journey that could easily fit another arc and few episodes, but with it being the finale meant they had to speed through it to get to the ending. It’s not terrible and it doesn’t ruin the show, but it’s sad to think what they could’ve done if given more episodes

So regardless of their quality, what are some other anime that squandered a cool concept/idea or in general are just wasted potential?