I think it's safe to say the Editor needs a bit of work done to make it really usable.  I've only been playing with that mode for about a week or so now (I prefer Creative mode, hmm, wonder why) but there are a few choices and possibilities for improvements that seem fairly clear to me.  Okay, more than a few.  It's a lot.  I'm sure that some have been mentioned before.     Separate Water from the Terraformer  (in the Editor and in general)
...or at least give water its own dedicated set of controls.  As it is, the fluid dynamics are pretty funky, and when we change the ground below the water line (expanding a bay for example) we end up with inevitable air pockets and bubbles that can really glitch out a swimmer.  It (kind of) flows on top, but not so much on the bottom, and there's not a good way to fix it when it goes wrong.
For the seas, the coastlines, and any spaces that are open to them, the tool should simply set the water level and let the water fill in everything down to the dirt/rock/sand that's below that level.  This would include tunnels, if one of us were to dig one from the ocean floor.  Be careful not to let it extend into the land itself, or you'll end up with deep caves being flooded.  (That's realistic, but not so much fun.)  For anything away from the coast, it should set the water level for that space in a similar way.  Doing this would allow us to easily create rivers or inland lakes.  Might be a cost in performance, but I think it's a worthwhile cost that could be improved upon later.
While it edit mode, maybe it can just show as a flat blue plane, then render out to look like water in play mode or when play testing. Editable Topographical Map (with 2 or more modes)
Elevation/Topography Mode would be a black-and-white map.  Darker shades are lower elevations and brighter shades are higher; I think that's the standard.  Allow painting on the map with "up" or "down" to raise or lower parts of the island without affecting the terrain type.  Different brush shapes and sizes and varying amounts of blur (smoothing) would be awesome.  Bonus points if it shows the main water level somehow (shaded blue if it's below sea level?)  **Tricky part: How do you handle structures?  Move them up or down?  Or let them get buried?  Have option settings on the object groups?
Terrain Type Mode would show what the ground type is at each location, and you could paint there much the same as you paint directly with the Terraformer.  Only faster.  Much faster.
Flora/Fauna Distribution would let us paint custom patterns for relative density of certain plants or animals (as a subset to what their total spawn amount would be).  Be able to paint an entire island of happy little trees.  Bob Ross would be proud.  (As an extension: Let us dial up or down the spawn values for specific items, including being able to turn them all the way down to 0.) Speaking of Maps: There needs to be one.
Give us a full map we can zoom in or zoom out, and set waypoints or spawn points directly on the map itself.  (Make them auto-settle to ground level.)  Maybe include an optional grid overlay or scale indicator.  If there's a way to export a hi-res copy of it for old farts like me who like to scribble on things when we're planning, that would be even better. And speaking of the Terraformer:  Give us ways to restrict the movement and the area-of-effect of that tool.
As it is now, it's very easy to end up with huge blobs of dirt, or rock, or even water, if you're adding using the tool, or to end up with unwanted chasms if you're using the remove function.  We need a way to lock it to a particular height, for example.  It could work like the "G" button does when we're building, only it locks/unlocks the direction the cursor can move.
The Area of Effect really should not go anywhere outside the cursor itself.  (For that, you'd probably need to add a vertical part to the "Flatten"" and "Smooth" functions.)  It's infuriating to try to manually cut a cave, only to go back out and find that Flatten has wrecked the overhead landscape from beneath.
Another nice feature would be if we could adjust the angle it's moving and affecting (for example, to "flatten" a cliff face). The Terraformer's Smooth function needs an "Unsmooth" option to balance it out.
Hey, maybe I want that part of the mountains to be really jagged.  Don't judge. We can select an area and mass delete.  Now we need the opposite.
Give us a way (maybe as part of the map idea up above) to mass repopulate a cleared area based on the island's randomization settings.  Would make it easier to redesign the topography and make it look natural again without having to manually place hundreds of trees, bushes, grass, etc.  As it is, if we lower the ground level, trees might or might not drop to it, but if we raise it, they end up buried. Extend the Show/Hide functionality to include topographical levels of the ground itself
Imagine being able to simply hide the top of a mountain so that it's invisible and can't be edited, then start cutting a large cavern into the middle of it.  After you're done, just reset the option and the top of the mountain is back.  Would make underground caverns and lairs easier to design and build.
Naturally, if this is active, then you would have to restrict any structures from going too far above the visible level, to prevent accidental collisions.
(And please fix the Show/Hide options, because if they're set to hide something, and another part of the island has to res in when you move, it doesn't always get it right.) Give us an "Apply Physics" button or menu option
It saves performance to let things hover in the air while we're editing, but we really need things to fall where they're going to go before the players show up.  It could be a single command that just applies gravity across the map, and fills in water where it needs to be.  Framerate would dip for a few seconds, but it's only an on-demand thing, then it can go back to letting things hover.
Personally, I would include an option to forcibly remove any mystically floating ground fragments the Terraformer might have missed. Give the option to show structures on the map
Pretty simple concept, maybe as part of the map editing above.  Let us select all, or none, or if we want to exclude certain ones (like maybe a hidden cave entrance).  Extend this to location names if we add any.  (Those could be added like a waypoint.) Let us manually set the time of day while we're editing
And make it default to the brightest part of the day.  If we want to see what it looks like at sunrise or sunset, we can change it.  The start time for the play scenario should be completely separate from this (and changeable in the settings). Lava?
Because scenarios on volcanic islands could be cool.   I'm sure there will be more, but it's late and I have to work in the morning.