Thank you for your great idea. While we cannot commit to a specific target version, we will consider adding it to our future internal backlog. If you can add further information about the context of this feature, please add it here so we can evaluate it more fully.
Note, that in views you can already present a "+" / "-" by utilizing themes (e.g. "Analyze (Grid)").
I think this is already possible in the 365 Add-in, isn't it? It's not particularly a + sign, but arrows.
In case this is implemented please treat the "+" as a format and do not preceed the name of the element with "+".