Widget that can execute other widgets?

There isn't a way for a widget to execute other widgets, however, a widget can execute an automation via the manual execution options if the goal is to reuse scripts/code.

would it be possible to use a IScriptablePlugin and somehow call controller.RenderContent and pass in a fragmentId?

A scriptable plugin can only render its own widget(s) and it would incur more plumbing: 

1. Implement scriptable plugin
2. Implement widgets
3. Implement scripting API extension to expose execution of scriptable plugin widgets
4. Use new scripting API extension to execute plugin's widgets.

vs Automation:

1. Implement automation w/ manual execution enabled
2. Use existing scripting API extension to execute the automation.

I actually wanted it to render UI components.

Running an automation from a widget doesn't prevent that. The only limitation is that the response from the automation is a string (so an interactive object cannot be returned) though that would be the same as rendering a widget from a widget. It's the same scripting APIs, just with fewer required expectations (automations can't generally assume that there is contextual HTTP request data).