@WD98 There are a few questions you should consider before coming up with the methodology. First do you need the terrain to be player editable? What sized worlds are we talking about? Also are we talking about smooth terrain such as in a typical game or Minecraft style terrain? Do you want height-mapped terrain or more complex terrain with overhangs and caves?
In general a whole planet can be stored as a single 32 bit number, a key to your procedural algorithms. Now if you want to add more variation in your terrain, you will probably want some extra parameters. In any case you can store a planet in not more than a few hundred bytes. This all assumes you don't care if your terrain is hand tailored to exact polygons. You can always just generate a random key and set parameters until you get something you like.
As for regeneration time, in my experience it's pretty fast, and by that I mean it can be a second or two. keep in mind that you only have to generate what you can see. If you are zoomed way out, you see the whole planet (or half really) but it's in low resolution. A single tri can be kilometers across. As you zoom in, of course you need more detail, however the closer you get, the less of the planet you can see, so you can render far less terrain.
If you put the effort in you don't need to store much on disk. You just need a good LOD system to make it all work. I don't use Unity myself so this may put more constraints on what you can do. But I've managed to get things working OK so I know it's doable. Right now my working planet is 8000 km diameter at about a half meter resolution. It maxes out at around 2 gig memory when running. However I'm using voxels, if you want to do height mapped terrain it will take far less memory.
And Last since the tiles are more or less flat How will I spawn tiles that are closer to poles and are curved? i.e. as you move across the planet the tiles spawning in front of you should be spawning in a spherical Order.
That's a tricky question. Ignoring Unity for a second you can subdivide a cube to the level you want it at, and push vertexes out to form a sphere. That gives you very roughly squarish tiles, but you will get a lot of distortion in places. I use an icosphere or rather a voxelized version of one. This gives you triangular chunks which is a bit harder to deal with, but you do get more even geometry.