If the number of levels are unknown, or just potentially very deep, then consider using breadcrumb navigation, where each item in said breadcrumb is a drop-down menu itself. E.G.:
Home > Plugin > Page > Subpage > Page settings > Confirm page
If, for example, you hover over "Plugin", you would get a drop-down as such:
Home > Plugin 1 > Page > Subpage > Page settings > Confirm page
And for other breadcrumb elements, where it applies, you could do the same:
Home > Plugin > Page 1 > Subpage > Page settings > Confirm page
- On mouseover/hover of the breadcrumb item, give the user an indicator of whether there are items below it;
- Repeat the current breadcrumb item to allow the user to jump back to that level, if at all possible;
- If items in the breadcrumb are filters, you might want to add an option to remove the filter from your results;
- The last item in the breadcrumb is the current page being displayed, always make sure this is not a link and doesn't behave as such to properly indicate this fact.
Also we want to display more than one plugin pages simultaneously.
Bad idea in my opinion, but if you really need to you could simply open the plugin in a new window/tab (also a bad idea) or position the two plugins side-by-side... which obviously leads to issues when the user's screen resolution is too low.
If technically feasible, allow a link to be opened inside a modal window that can be dragged around. But keep things simple, usually you don't want to overload users with too much information. If they really need to, they'll just have to open the plugin in a new browser window.