Option for "whole note" = "fill the bar" for any meter
In odd meters such as 5/8, if a voice sustains a note which should fill the whole bar, the only "standard" way to notate it is to tie two notes together, which looks very clumsy.
A better practice is to simply use a whole note. This same practice applies to any odd meter. If a note needs to fill a bar, regardless of the meter, a whole note can be used. We are used to seeing this already for whole rests, so the meaning for musicians reading the music is obvious.
Currently this can be done using workarounds in MuseScore, with the following tricks: changing the note head manually and hiding the stem. This works in all cases except when the whole notes need to be tied to the following measure. In that case, fortunately MuseScore allows tying over rests, so in 5/8 a half note can be entered, changed to a whole note head with hidden stem, then the 8th rest can be made invisible, and the note can be tied across the hidden rest.
There is a price to pay for these workarounds:
(1) layout problems - when these hacks are implemented, the layout can behave oddly
(2) MIDI export problems - notes tied over rests are rendered with incorrect lengths (or you could say they are the "correct" lengths since they reflect the actual notation, which has been fiddled with for visual reasons).
(3) it's time consuming to change everything manually.
The software could obviously do this for us. A simple Style switch could turn on a "whole notes fill the measure regardless of meter" option. Then in 5/8, we can just enter a whole note, and MuseScore will treat it with the desired value according to the meter (the same way default whole rests work).