I play this game almost entirely to build stuff. Here are some things that would make my (and hopefully many others) time spent building much more enjoyable and streamlined.
**Make paint work the same as armour when new blocks are placed. **
Currently a newly placed block will use the last selected armour, but will be unpainted. This should be changed such that a newly placed block uses the last selected armour and paint. This would save me (and probably everybody else) an ungodly amount of time and clicks.
Make the replace tool useful.
So currently the replace tool can only be used to replace modules of the same size and will replace ALL instances of that module. This is quite literally useless. Instead, a working/useful replacement tool would be able to replace the selected blocks (AND ONLY THE SELECTED BLOCKS) with modules that fit in that size. For example, selecting 3x1 filler blocks, I can these replace these filler blocks with a command module or whatever other modules might fit in that selection.
The reason this is important is because I (and I assume many others) usually shape my builds with filler blocks and decoration. It is not uncommon to have decorations 3+ layers deep. This means that if I want to change a block with a decoration on it for whatever reason, I have to re-place all of the other overlapping decorations in their specific order. This is a lot of clicks for no reason, and a working replacement tool would solve this issue instantly. If this is not clear I can provide an example in the comments.