Serverless. I started playing around with some of the open source frameworks that exist (mainly OpenFaaS and Openwhisk) and found the experience pretty cool. Certainly, serverless doesn't work for everything and might be a bit overhyped, but it's a very pleasant development experience at times, especially with OpenFaaS imo. I also added a crystal template for OpenFaaS (https://github.com/TPei/crystal_openfaas)