A relevant thread: Limit Application Memory Usage with systemd - KDE Blogs
Limiting resource usage like that, is probably a more realistic use-case than freezing a game to compile code. That sounds like something one might imagine as an example but maybe 3 people would ever actually do it 
Likewise, raising the OOM threshold (for apps that you know might have high memory usage but you really don’t want them to be killed first if RAM runs out) or setting an application’s CPU affinity (‘pinning’ an app to certain cores), are more common use-cases which can be implemented through cgroups.
I guess this is a big question about something like this - of the hundreds of available features, which among them would be really useful to implement? And then, would the interface of System Monitor really be the right place for them?
I really like the idea of exposing cgroups to the user in a GUI, though, it’s a super powerful and super under-appreciated feature in linux, with a super 1980’s interface.