allow me give a brief example then ill go more indepth with the aspects the mod would have.
1st. the mod would add a map (image) - which allows users to 'see' earth and where they are on it ie if they are in boston the boston worldspace then that would be where they'd be seen.
2nd. i imagine itd work like the map in older fallout titles did, bassicly with this mod users could select where they wanted togo on earth and if there is a worldspace there then they'll be able to fast travel there. so if a player has nuka world they'd be able to select nuka worlds' location on earth and fast travel there.
3rd. (optional) Restrictions/travel options - this part id imainge would be harder so thats why i put it as optional as i dont care honestly if this was added. bassicly if a player wanted to fast travel to far habor they'd need access to a workin boat for example, other benifits to if this was added would be travel times could be shorter if the player has a better transportation then by foot. of course there would risks of using different transportations such as cars would restrict them to roads and trains id imagine could have random encounters where the railway is blocked and the player would have to remove the blockade like in the game metro. (i doubt itd be possible but perhaps if it is possible the mod could read OSM data, to decide travel pathing?)
4th. (optional) a Zoom Fuction - on the earth map only worldspace travel points would be visable ie boston, nuka world, far harbor would be seen on the world map this would hopefully help make it easier to make the mod.. (whoever if anyone decides to make this mod maybe make it where at max zoom out only worldspaces are seen then if zoomed in on boston you'd be able to see the orginal boston map??)
these are just some ideas im no modder tho i have messed around with putting my hometown in fallout 4 using the geck but what stopped me from continuing workin on it was the lack of a framework for adding new worldspaces to the game.