Happiness usually isn't a singular end, what problem do you have that an app might exist to address (in a perfect app world of forms)?
Like I found my perfect calendar app that clicks and helps me be proactive and know what the hell is coming at me or what I need to know to keep things going. That makes me happy(er) that I have my schedule organized and in a sustainable system, the happiness is incidental to that end being met
Custom Card Deck app (iOS). I just want to be able to add a bunch of custom images as a deck of cards (front/back), then just draw a random card from that deck, and have multiple card decks like this that I can switch between and draw from. Doesn't even need to keep a hand of cards, just draw one random card at a time from a deck.
There was one app that kind of sort of did that, but the interface was pretty janky and it didn't seem like it worked that well, plus the logo was ugly as hell.
Maybe not for me, but for the average user: an app that allows you to connect to any (consumer) router and configure it with a streamlined iOS UI.
It would be quite the undertaking to develop as there aren’t any standard APIs, the only thing I can think of is having something Selenium adjacent to drive a browser in the background (which is so unreasonable).
So many times I’m tasked with helping friends/family set up their all-in-one garbage router from the ISP, and their web-based UIs are atrocious and time-consuming to work with.