It would be a huge undertaking, but a Fitness and Health tracker / aggregator that could replace Google Fit and the likes.

I really can't bear how Google, Apple, Samsung, and all these big companies are the primary holders of our most intimate information. I've put some measures in place to limit who gets what, but it would be a huge boon to be the sole maintainer of my own info.

The problem is that the various apps and devices which report data won't immediately support syncing with a FOSS upstart...

The app I use for grabbing my weight and BMI can only sync with a few other apps. The app I use for calorie and diet tracking can likewise only sync with a few apps. They happen to have Google fit in common, so I use that as an intermediary to transfer weight to the calorie/diet app. All my steps, exercise, and sleep stay in Zepp, separate from them all.

It sure would be nice to have one service/application to rule them all and a secure method of storing one's own personal information without having to give it to the tech companies. Sure, use one of the many cloud services but encrypt all the data so that they can't steal it. Yadda yadda.

One can dream.

Obsidian.

Markor is a great open source markdown editor for android, but I wish we had some decent WYSIWYG options, like obsidian, typora, etc.

One of those "smart" alarms that monitor and graph your sleep. E.g movement, sounds, snorings, sleep talking etc.

At a minimum one that wakes you up in the 30 minute window of your lightest sleep phase

SleepAsAnAndroid as well a broad support for generic smart watches

My banking apps. They are the only reason I can't fully de-google myself.

Nova launcher - there isn't a good one for one FOSS replacement. Every launcher I tried from fdroid has at least one shortcoming (if not more).

Discord. I hate that premium costs so much and all the ads they put in place to sell useless junk features.

Google maps. So open street maps but with reviews like maps has. A few days ago people suggested apps, but they lack reviews. I disagree that they are useless.

A really good launcher!

Also, weather app!

I can't really think of anything now because Android FOSS apps ecosystem is really good. What I want to suggest is contributing to already existing projects sometimes. It's faster and just another thing you can do to help open-source ecosystems

Google lens.

I love point and translate. Faster more pin pointed reverse image search is cool too.

Right now im looking for an alternative to the Google Maps Timeline. I know there is OwnTracks but I dont think that everything has to be hosted on a server somewhere (especially when all its saving is a timestamp and a coordinate, its not like that takes up alot of space)

Basically just your own location tracker and then the option to see your own history displayed in a map e.g. where you have been on the 02.july.2019 at 11:50.

A modern replacement for OpenScan. It's workable, but some features don't work on Modern Android, and a good Scanner app is probably something most people could use. Could look at Adobe Scan and Office Lens for feature inspiration.

Poweramp

The way it handles its own audio engine is pretty much the best available.

Obsidian.

There's lots of FOSS music players, but none of them have a volume slider / preamp. The Android volume slider is always either too loud or quiet so I have to make fine adjustments using the preamp in JetAudio. If someone could add that to an existing music player that'd be cool.

I'm sorry, I don't have any specific suggestions for you, but I am wondering: is there no open source app you yourself wish existed because you would need it?

Working on an open source app because some else (and not you) needs it, is not a good way of staying engaged and caring about the solution. Being the user and target of a project yourself is usually a much netter way of caring and proposing something tailored to at least one individual, maybe more.

Of course, if you are looking for a programming exercise, go for it, but then you don't need ideas, you can reimplement something which already exists, perhaps which you like, but in your own way. But if you want to have an impact in the open source, it starts by needing something which you don't really find anywhere and taking matter in your own hands to fix it :) this is not meant to disincentivize you, quite the opposite! I hope you stay attentive to your digital ecosystem to see which holes can be plugged :)

I maintain a private list of ideas I just think of as I go about my day, of things I would like to write/create for myself and while I won't be going through with all of them, I hope to be able to pick up one or several of them whenever I have time. I can through some ideas here, not as a hint that you should do it (I'll probably do them myself regardless), but just to inspire you, maybe:

• I am subscribed to a teachable program which has no app and the program is just static information. I want to pull it all and represent it to me offline, not requiring internet to manage my progress. It is also intended to help me archive what I paid for and not depend on the goodwill of teachables to allow me to continue access the resource.
• an RSS feed manager which uses embeddings to automatically organise the content by topic rather than by source.
• an anki plugin to highlight content in the browser based on words from anki that I have and have not learned, to improve my language learning and reading ability.

I have a few more, but this should give you some hints, I hope! Good luck!

Nice question, you've got a lot of answers, most being major projects outside the scope of an individual, still, interesting pain points.

May I suggest you edit your OP with a list of viable options for individual devs or small teams to try ?

I think an open-source general device benchmark would be cool. Including CPU / GPU / Battery life metrics. As far as I know, everything that does this is proprietary.

Symfonium. Don't get me wrong, I like Finamp, but it just does not come close to the amount of features that Symfonium has.

Support for all the hardware so we can quit using garbage Google-provided Android on every single phone instead of just the half-dozen or so phones some of these Free Android builds support. Amazing that I can install Linux on every single freaking configuration of PC that's ever existed with a very tiny amount of systems not having support for all of its hardware even if said hardware has never been freed or even officially documented, but not with phones.

The damn things will still be a privacy nightmare because your cell signal tracks you everywhere you go, but at least we'd have a Free OS for everyone's phone.

A video editor...

Any image/video editor in Android. I've been trying Image Toolbox, but sometimes it doesn't work or crashes :(

I can't find an equivalent replacement for Musicolet. There are plenty of open source music players out there for Android that just don't have the little features that Musicolet does. Such as multiple queues, lyric editing, metadata editing, format conversions, stop after the track finishes, easily reorder songs and clear queues, etc.

Meme Generator

A nutrition tracker where you can enter what foods you eat into a small database. And then when you eat meals you can check those foods off in order to calculate all of the nutrition facts consumption per day. And it could be expanded even further by adding graphs and reports such as Weekly, Monthly, or Yearly.

Could track Calories, Vitamins, Minerals, and other specific nutrition stats. Most nutrition apps I've seen only track Calories... Or don't have accurate nutrition applied to specific foods as it is generic. Letting the user add the food as a item in a small database would give the user more control of how the stats and reports are calculated.

Could be helpful for some to see their intake and then figure out ways to change it to become healthier.