I would like to make a game somewhat similar to a mixture of Far Cry and Minecraft - basically a playable version of the movie Cast Away. You're alone on a tropical island, do what you have to to survive. You'd be able to hunt, gather resources, make tools, and build shelter. Ideally there would be a day/night cycle and a seasonal cycle as well. It would also be nice if I could make it first as a singleplayer game with the option of adding multiplayer later, but this is non-essential.

I don't really want to start from scratch, I'd rather try to mod an existing game if possible.

I don't have a huge amount of game development experience, but I do know both C++ and Java. How should I go about getting started with this?

Not really a clone though. Much less building based, much more survival based. Definitely not using blocks.

Do you want to make it 3d? (I think the answer is "Off cource" but im not sure )

Yes, definitely 3d - I'm not sure if there would be any way to make what I want out of this in 2d at all (I'm also not sure if the game would even be fun, but it is still my ideal game so I would love to be able to make it).

I am aware from previous game attempts that resource creation ends up being the hardest part, so I am trying to avoid that as much as possible.

How easy is Unity/C# to learn? And what kinds of resources as far as models, animations, etc does it come with? And outside of that how readily are basic resources available online (i.e. stuff I could throw in initially as "programmer art" and replace later if necessary)?

I discovered a great aspect of Unity today, which is that it comes with a premade island level that I could definitely use as a starting world to work with. So it would just be a matter of scripting things to work the way I want them to, and learning the mechanics of how various design aspects would work exactly in Unity.

For example, a feature I would want to have is the ability to cut down trees using a crafted axe of some sort. So I would have to figure out exactly how to replace the tree with a stump upon finishing cutting it, and hand the tree itself over to the physics engine so it could fall properly.

I want it to be HUD-less, so that will make things easier.

Another feature I've been mulling over how to implement would be the use of ropes (craftable via vines). I would want the player to have the ability to tie said ropes to different objects in the world, so, for example, the player could tie ropes between trees and to each other to make a hammock. I'm not sure exactly how I would script this though, since the exact appearance of the knot/rope would vary depending on what it was tied to.

I haven't read any of the manuals/tutorials/etc for Unity yet though - I'm assuming once I read through some of it a lot of the features I want to add will end up being pretty straightforward. I have a feeling a lot of the hard parts will have to do with the various modifications the player can make to the world - i.e. if the player wanted to dig a hole in the ground I would have to figure out a way to have the terrain be able to be modified.

Also want to have a realistic ecosystem, in which vegetation can die based on the amount of water/light it is receiving, and where if the player over hunts a particular species, it can go extinct, or alternately one species could end up over-reproducing and take over.

D33ts that sounds like a great idea! I would def. want to help you with that if I knew much myself! Def. take some time, sit down and just start looking through tutorials, I've been wondering how to do an interactive ecosystem as well, but no clue how.