Is there a way to do trait based placeholders

Your question isn't completely clear are what you're trying to achieve, but take a look at creating components.

hi! for the select model you may use editor.Modal and as it's content, inject your desired logic for choosing and configuring your widgets. then you can inject the result widget using yourComponent.components(yourWidget) to fill the content of block with your widget. for the ajax rendering part, you can define some logic in the view property of your component type definition as well.

Sorry Let me explain

take a cms widget such as {{widget name="test" type="\Php\Class"}}

Concenpt is to drag a block which opens a widget select model to choose a widget set the options and hit add that generates such as above. This needs to the ajaxly render to canvas, but leave the html with the above widget.

Ive done something similar for asset management, and got normal placeholders working but the variables in that widget may have different values