This suggestion is pretty simple, and will be in a few main categories.
Defenses
Guard post: this would simply spawn an NPC guard at the post. The guard’s level would scale with the guard post’s tier.
Defensive tower/post:this would start out as an archer, would go up levels and turn into guns as it goes up in tier.(would use different model than the current ones)
More tower types. This would include some between-wall towers with an entrance at opposite sides for the lower tiers, because it is unfair only tier 7 gets all the customization.
More gate types: a large gate, and a taller large gate with no rim around it to let ships through, lower tiers would have reinforced pillars to hold it, higher tiers(4+) would be given small towers at the ends.(would probably use a menu popup to clan members at the island)
More wall types: just some raised/lowered walls, and some staired/slanted walls.
Decoration
Bones: bones and skeletons of humans and various animals, including dragons(dragons or large beasts would be found inside large dirt piles with their material variating based off the terrain their placed on, or they can just use ground/coast/rock like terrain do)
Houses & other structures: small, medium, and large houses with interiors to decorate(with no-interior versions, too), Ruins & temples, and other stuff like those towers with the bells at the top or some storage buildings.(if the devs want to, they can make citizen, thief, and guard NPCs spawn at the ground near houses and walk around)
foliage/rocks: plants like cacti, trees, bushes, & moss, as well as rocks of variating sizes, including stalagmites.
more interior/exterior decoration: umbrellas, tables, shelves, & other stuff like that you can use to decorate.(especially since the barracks can’t even be a proper barracks yet, as there aren’t even beds)
Civilization
More shopkeeper variants: this would be pretty simple, a decoration-less and fancier version of the merchant stands, and a small decorated house with the merchant’s icon on a sign, and has a shopkeeper inside, with some decoration depending on the shop type
Fruit farming: the fruit would need a farmer or player to collect them, and would give the owner of the island or the picker of the fruit the fruit, would be on a plot where you place the fruit in(would be bushes, vines, & trees depending on the fruit type)
Fisherman: a fisherman that will automatically give the clan galleons, and give a small boost to fishing for the clan.(or it’d give the island’s owner some fish automatically like ship crews do) The fisherman would require a castaway though.
Terraforming
Caves: small, medium, and large variants of caves with raised and sea-level variants, would have open-top versions for ladders or stairs.
more raised terrain variants: short, tall, thin, and small raised terrain(there should be no need to explain this)
Bridges: wooden and stone bridges of different designs, would be segment pieces of normal and raised variants
Other terraforming structures: cliffs, arches and thin tall spires, of varying sizes
other
More NPC customization, at least something like deckhand customization(would be probably based off the clan theme colors), also make a toggle for the cannons and wall barriers on your base(cannot turn on wall barriers if defenses are down), also make raised terrain snappable
reason to add at least some of these
1: will allow for more creativity & customization to islands
2: allows people to better defend their bases while they’re away
3: will give some more incentives to build at islands