Personally, not much.
Garry’s Mods’ workshop needs better organizing, I think the duplicates, films, saves, and addons need to be organized for exclusion. This way users could cancel out what they don’t want, and keep what they do want. There are so many duplicates that’s not even funny…
2. Author care
Authors also need to be protected on the workshop, users constantly ask questions that annoy the author and with no way to block these questions the author becomes unresponsive.
Authors should be compensated for the work. Authors put a shit ton time and effort into their work and get nothing in return. Example: Adfly.com, one idea is to put Adfly Link in their description so the subscriber can click it and give the author money.
As for as game modes, I don’t play that much online. We just need people to be more creative.