I wonder what you expect such a popup to work like.
While NP++ comes with only one window per instance which has only one visible view, ST allows many windows with many groups which can contain many views.
In abstract words: While a tab switcher popup in NP++ needs to display only a list of views, a switcher in ST would probably need to provide features to switch between all windows/groups/views of an instance. 1 level vs. 3 levels.
Would alt+tab show a popup with views of the current group only?
Should it display all views of a window?
Should it display a list of groups to switch to?
The complexity of ST's UI arises a lot of questions and possible solutions/workflows, which then would need to interact with the plugin API as well.
I use BetterTabCycling plugin for instance as I prefere alt+tab to only cycle through views of the current focus group.
Another question is: What value does such a popup have? You can't read any content being outlined in a small preview and the list of open view's in a focus group is available via tab menu as well. Tab cycling is quick already.