I think one thing GitlabCI lost in taking inspiration from Concourse (amongst other tools!) was the centrality of resources. It wasn't obvious at first why this is so important, I and most others fixated on the other visible differences (containers for everything, declarative config, pipeline view etc).

There are a lot of features that don't need to be added to the core because they can be resources instead.

For git-triggered builds, I use the git resource. Periodic, I use the time resource or a cron resource. S3 triggered, I use the S3 resource. Triggered on release of new software to PivNet, I use pivnet-resource. Triggered on a new docker image, I use docker image resource. And so on.

Any resource that has a 'get' operation can trigger a build. 3rd parties can add triggers without needing to avoid interference with other resources, because every operation is isolated.

Disclosure: I work for Pivotal, which sponsors Concourse.