Laying out scores nicely can be harsh and frustrating.
If I take a little example.
When I have a repeated measure that must be repeated more than 2 times, I like to have an indication "4x" above the repetition sign (MuseScore doesn't add this automatically, so it is up to the writer to add it.)
I like to have that indication right on top of the repetition sign.
I enter this on the last element of the measure as a "system text". And it is positioned by default relatively to that element.
So to have it right on top of the repetition sign, I have to move it manually.
But, if in the layout process, I make some change (e.g. decide to add a line return for better readability) the indication is no longer on top of the repetition sign. And I have to re-place manually in all the parts.

The same applies to many other examples: tempo changes, rehearsal marks, custom palette symbols, ...

With layout aids this whole process could be definitely eased.

Proposed aids

Anchors

Instead of having all those elements positioned relatively to the note they have been added on, there could be other anchors options, like start of measure, end of measure, ...

In my previous example, "end of measure" would be definitely the right choice.

Guidelines

Being able to place guidelines on the score in order to easily align elements together, with possibly a "glue"/"snap"/... function to attach/attract these elements to the guidelines when thy are move close to the guideline.

With these guidelines, the rehearsal marks [B] and [A2] could be easily aligned on their left side.

Aligning elements together

When several elements are selected, having the possibility to align them together, be it vertically or horizontally would help laying out a score faster:

With these actions, the tempo text and the custom symbol could be aligned vertically on their centre in one click.