Good question. There is no specific theme as you'll need to create a solution with a use case of your own. You'll just need to follow the main requirement wherein you address one of the categories:
A graphical representation of data through visual components such as maps, graphs, or other visual formats (e.g. tools that display trends or patterns).
Solutions that collect, interpret, and present data (e.g. tools that identify relationships or detect anomalies).
-Data and Machine Learning
Solutions that use pre-trained models on AWS Marketplace and/or train machine learning models using a specific dataset (e.g. predictive analytics tools).
Also, don't forget you need to use at least two AWS services and a 3rd party data set from AWS Data Exchange.