So, a new island is raised near the castle, you'll find some gifts and a workbench to construct your own paradise...

you'll find the location on your pipboy around the castle and you can travel direct to this paradise...

but, take care and be prepared, most often the nice places are occupied by nasty owners, you'll needing to gain the acces of course...
you also can acces by the soil, i maded an passage way for the pedestrians wanderers

I let this island as empty that it's possible, to let you totaly free to make what you desire, i think it's enough spacious for a nice place to live...
i had drawn all the navmeshes that be useful...

no dlc are needed, as my other mod, i make them totaly without other mods or dlc, had just used the fallout4.esm.

you're free to delete my welcoming commitee with the CK if ever you don't like it... but it can be boring without :)

for those who encounter the can't sprint problem, add this in your fallout4.ini :

Under the [gameplay] section in fallout4.ini (my documents/my games/fallout4/)
(if there is no [gameplay] section there, add it)

add the following line:

fPlayerDisableSprintingLoadingCellDistance=0

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NEW VERSION (2.6)

As suggested by GreekRage, i made twin islands,
the main island hasn't change a lot, to preserving your previous builds, just a little bit changed the beach borders, only less grass, and the landscape underwater, for better viewing with more kelp and better slopes where it was possible...
It increases a lot your build space,
as previously, i hadn't put any builds like bridges or something like it, to let you totally free to do what YOU want...

In addition i also put an unique .44 on the workshop workbench, i called it The Howitzer, powerful for a pistol, tried to not make it too overpowered, but everybody have different appreciation of the overpowered word :)
you will just need to improve it for scope and the breech, if ever it's not enough overpowered :)

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Previously, VERSION (2.5)

i think i found the solution (maybe, i hope, lol), just had needs to set workshop in the encounter zone form, wasn't set automatically, don't know why, had tested, sended a new settler from another location, she came and assigned herself to farming, so i think it's good now for the new settlers, haven't see the supply line coming yet, but it can take a little bit time i guess...

Previously :
slight changes, terrain details like smooth a little bit the build aera, remove a little bit of the grass...

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Added images for one of an exemple of what you can build on my island, but it's just an exemple, no kind of build pieces you can see in the images are included in my mod,
i used this huge mod to build this kind of "house" from DDProductions83 : http://www.nexusmods.com/fallout4/mods/7790/?