Honestly, I could try making you one. Last year, I was trying some React Native and did some apps to study the technology. Can't guarantee I would make it my priority though, since I don't have a lot of free time.
What do you have in mind? You add an address/xpub and it just shows up the balance (and maybe last transactions) in the screen?
It's not an standalone app and it's more focused on notifications on new transactions, but there is an Telegram bot I use called @MagnumNotifierBot which also shows the balance of an address whenever you want to.
Widgets I could live without...
But realistically something that lets me add public keys to watch wallets for at least Bitcoin and Litecoin.
Just about every app out there has trade this, sell that make accounts etc. But nothing I've found for specifically watching addresses that's not overly involved with a bunch of useless shit. Which is still nothing that I've found.
Anyway...
Most apps run in the background nowadays, so I'm not really worried about a few extra power draws or whatever. But maybe still have an option to turn that background option off for others, if you make it a public release.
But something that maybe runs in the background to check addresses for transactions say every 2 hrs and does a push notification when it finds one wether it's a withdraw or deposit.
But when the apps opened, it forces said update check and displays on screen accordingly.
But the GUI being something that shows a generalized interface that shows said address balances and maybe when clicked. Open a transaction page to show what's transpired on said address chains... so standard wallet stuff I guess.