Backstory: If you count all of shopify's customer's revenue together, you'll find the second largest eCommerce platform, behind Amazon. Shopify recently launched their subscription api: https://www.shopify.com/partners/blog/shopify-subscription-apis, you could get in early and launch a plugin around this new offering.

The Idea: A shopify subscription platform.

Deep Dive: You're probably thinking which one?? You could go down a few routes but it's best to do some research. Check out a few of the current apps on current plugins and other subscription platforms:
1. ReCharge - https://recharge.partnerpage.io/integrations/
2. BoldCommerce - https://boldcommerce.com/enterprise
3. Subbly - https://www.subbly.co/

From being in the subscription space, one of the often requested and overlooked feature in almost every subscription platform is box/ordinal tracking - "How many boxes has this customer received?" Seemingly simple but it gets complex when they have multiple subscriptions, purchase one off items and the core system has no way of tracking this. This is a feature that appeals to larger customers as well, those that have already established product market fit and are looking to improve retention. Often, subscription companies will send a bonus or a thank you item after 5-10 boxes shipped. 

Validation: Checkout shopify's stock price, they're not going anywhere. As the second largest e-comm platform, you'll get access to hundreds of thousands of shop owners by listing in their marketplace. 

Monetization: Ideally you'd charge a monthly subscription for your app. By appealing to subscription based businesses their revenue is fairly stable monthly and they can predictively pay a monthly subscription for an application that's providing value.

Steps to Launch: If you can't find an idea to start with, start down the content route. Build small widgets that solve small problems with shopify's api and post them public. Analytics is always a great place to start as platforms like shopify never seem to have a perfect set of metrics for everyone.