Farm Cave - Option to make it available or unavailable; (default - available)
Sure. Why not? If folks want to make their game harder, I have no problem with this.
NPC Birthday - Option for you to lose friendship hearts when you don't give a gift to a NPC on his/her birthday, from 0 to 3; (default - 0)
I would never use this system. It makes every NPC really passive aggressive.
Sprinkler - Option to make Sprinklers work on Garden Pots; (default - disabled)
I've never understood why this doesn't work.
Season Lenght - Option to change how many months a season will last, up to 3. Will also spread NPC birthdays and Festivals dates; (default - 1)
I think that the season length and the time needed to grow crops are pretty well balanced. If you change the length of the season, you end up planting 3 times as much of the same stuff every season... but you should be advancing to new seasons and new ingredients sooner than that, if you ask me.
TL;DR: I wouldn't use it as I think time moves at a good rate already.
Artifact Spots - Option to change Artifacts Spots to either have a chance to dig artifacts based on location, or being totally random; (default - location)
Random... as in you can now get
ANY artifact from
ANY artifact dig location? While I see how this could be seen as an advantage... in some cases it would limit you. Consider when you only have a couple artifacts left to complete the collection, if this option were on you would have to hit every artifact location and just hope that you got the artifact you needed. Without this option, however, you could target areas where the artifacts you need are located. Basically, it could make getting a lot of the collection easier... but getting
ALL of the collection harder. I'm not sure I like it.
Time Flow - Pretty self-explanatory, this will change how many real-time seconds are necessary to pass 10 minutes in-game. I'm not suggesting this one just because of multiplayer, I'm starting to feel the game becoming more like a speedrun than a relaxed experience, even when I automated a lot of things in the farm. I guess that the default cycle wasn't so bad at first because there wasn't so much content as we have today.
While I do feel rushed quite often, the compressed time that the game gives means you're not taking a real life year to progress through an in-game year, you know? This is, in a sense, just another way to slow the game down, much like the "Season Length" part of your idea. I'm not sure it's necessary. It makes you have to choose between what to do options instead of everyone doing the same things, you know?