So I started a new world a few months ago, and while it has lot of natural generated structures to explore(mostly late game, since there is just a lot of Ocean moments), I am out of ideas to do.
Mainly because the most of the world I have explored has been underground, or sailing oceans. But despite that I lack ,certain things like a proper enchanting setup, because I haven't encountered any cows(and Horses/Trader llamas seems to expensive to breed for just getting leather, and I haven't found leather in looting chests or with fishing, only a book to make enchanting table itself).
I have build my base, and decorated it.
I even build some primitive farms like Fish farm, chicken farm, and 80% of Cave spider farm, I just need to build a better killing chamber for that one.
My spawn island is almost located next Ocean moment, and Guardians spawn when I am on Island, so there is a potiential Guardian farm and it is not something I have done before, but it requires good preparation.
As you already mentioned, you need cows. You will find some if you spend a little time above ground in grassy areas.
You might also want to find a village and secure it from mobs so you can trade with the villagers.
Potion making is a useful pastime because you can do a lot more safe exploring when you have water-breathing potions, fire resistant potions, etc. With invisibility potions and water-breathing potions you can enter ocean monuments with impunity and kill all the elder guardians without getting attacked (just remove your armor beforehand). Having such goals in mind gives one motivation to go out and collect the necessary ingredients for the potions. You will need to go to the Nether to collect some.
You can work towards killing the ender dragon. You can search for a woodland mansion and clear it (that is something I haven't done myself yet).
You can experiment with redstone contraptions. You can build more automated farms. You mentioned a guardian farm - I just recently drained an ocean monument and built a guardian farm using the tutorial by the YouTuber Wattles. It was very time-consuming, but at least I have an endless supply of prismarine.
There are loads of things to do, but basically you should do things that are fun for you. Not everyone enjoys the same things. There are a lot of great YouTube videos out there that have something for everyone.
I was thinking of stealing villagers from a Village, and breed them up. as I don't think there is any villages nearby my base.
I have 2 hearts of the sea, but only one nautilus shell, so water breathing potions isn't a must have as I would the potienial of having two functional conduits taking down the ocean moment, when the time comes.