sorry to say but the features can't be "auto"-updated.
A feature.xml is more like a repository and is kind of static, therefore you would need to register a new version of the feature.xml again
with a feature:repo-add
What can be done, though is to make sure you connect Karaf to an OBR (Karaf Cave for example).
Now when you install or update a certain bundle it automatically can pull in new requirements, updated external bundles etc.
Usually you want to have a stable enviroment for your application and not a continous "re-deployment" unless you are talking of a CI/CD server.
In that case it usually is best to trigger the bundle update from the outside.
Either from a "shell-script" with bundle:update or via JMX.
JMX can be "proxied" through to a REST call via Jolokia.