I'd really like to see some data visualisation in Back in Black.
The Birdy (https://thebirdy.com/) is a website for keeping track of spending. It's got graphs, streaks, all kinds of goodness - the kind of stuff I'd like to see in Back in Black. It's had decent reviews in the usual places (Lifehacker etc).
What I'd like is for BiB to support exporting data to The Birdy. That way, I (as a customer) get the data visualisation I want, without having to wait for you guys to write it. Better yet, it's on my 24in PC monitor, rather than my 3.5in mobile device, so I can get both a large and detailed view. Best of all, it's already written; I just need a way of getting my data into it, and that shouldn't be too hard.
From a business perspective, this adds value to your app for your customers at minimal cost to you. More value to customers == more sales, as you'll start attracting the marginal customers who weren't quite prepared to buy your existing app. Plus, it lets you concentrate on improving the client, versus supporting both a client and a website. It's a little like how you outsource your support website to Get Satisfaction, rather than build/maintain your own support website.
On a final note, it'd give you an instant device-sync work-around - don't sync between BiB clients, just have them all update the same Birdy account.