On my channel, I have two alerts set up for the same event. One via Streamlabs and one via Streamelements. One pops in front of every source in the scene, the other pops behind the webcam.
This way, I get two animations per alert, and the one that's behind the webcam is fullscreen, which allows for some really cool effects. Paired up with a green screen this could mean, for example, a Super Sayian alert etc.
There IS a way to do this using Streamlabs alone, but it requires setting up the alerts in two different themes, and using the secondary theme's URL as a browser source for this to work.
Let us add more than one alert for each event, or, have multiple alertboxes in the scene, with the option to point each alertbox to a different theme...