Tempo Line
Adapt the TempoChanges plugin to use a line similar to a pedal line, but that can be manipulated like a hairpin to set the tempos visually. Tempo values would not be seen in order to avoid clutter. The tempo line can be toggled visible or totally invisible, not just greyed out if View>Show Invisible is set, but not visible at all. Again, the idea is to reduce tempo markings, lines that are not actual pedals, hairpins, dashes etc. Right clicking on the tempo line would bring up the TempoChanges plugin dialog or a similar dialog, but this would only be to confirm values although they can also be set from there. Tempos would mainly be set with the tempo line, confirmed or fine tuned with the dialog, and then the tempo line is made to disappear by toggling.
Comments
The plugin can only do (a subset of) things that MuseScore itself can do. It thus can't invent new elements.
The intention is indeed to include this functionality natively at some point, similar to how cresc/decresc lines work indeed.
But given the plugin as a workaround so far it has risen high enough on any volunteers coding list to actually make such a change.
See #9383: Rubato: ritardando and accelerando, one of the oldest still open issues. Unfortunately it didn't make it into this year's GSoC