Struggling myself with inset maps. It is possible with the vp argument of print.tmap, but requires a lot of trial and error:

1. tm_minimap with shows an inset of a world map, either a projected 'flat' one, or a globe, like mapsf does. This is already implemented for the view mode, but not for plot mode.
2. a more general tm_inset, which takes any (grid-based) plot, and places it in a map component. In that way it is processed in exactly the same way as other components, like legends and scalebars. The user still has to provide a height and width, and specify the position. Use cases: inset maps (also like this tmap_arrange -> patchwork-ish functionality #1046) and ggplot2 plots.

What do you think @Nowosad and @olivroy ? Any additional ideas?