Off the top of my head:
I'd like to see functions to, weld 2 blocks that are overlapping or apart, a similar fill(not only to fill a volume, but also to reconcile parts that are not precisely touching and bring them in-line so they can be kept or merged, an ability to morph placed blocks, , real scalable curved blocks, the ability to re-align/move the mirror lines and utilize multiple mirror lines simultaneously, and set the core block manually.
Additionally, a better UI for, and an optimized ways of applying scaled ship presets, instead of the not documented way of pasting to the prefab section, and attaching that way... it's all very unintuitive and difficult to explain.
Having a setting to ignore block clipping would also be a good feature(considering the faction ships clip extensively, I can't see it being a balance issue).
And TBH, creative should have a ton more features. Fighter, item, custom station, as well as crew creation with an interface similar to a free turret factory with a more complete control options including an item duplicator...
Additionally, resizing and panning in UI element windows would fix some problems we have such as the click through, and the inability to see some elements that go outside of a frame or even the fullscreen element.
Idk about allowing scaling in the UI element, but I can't see much reason not to beyond rendering issues... that said, it's not all that hard to deal with.
A dps calculator in the tooltips and an overhaul of sorting capabilities for the inventory, saved prefabs, and any other things that end up in lists... sorting really needs to be customizable for it to be usable in this game as things get WAY out of hand, really quick.
Maybe a toggled lock for items so they can't be accidentally jettisoned, fed to a research station, or sold...
Anyway, that's just my short list I suppose, in case anyone was asking.
;D