Slide between notes (playback)

I know this topic has been discussed as far back as 2010 but the discussions I have seen haven't really hit the nail on the head.

I am looking for a specific kind of playback modification whereby two adjacent notes are linked together into one single note with a change in pitch part way through.

Playing two notes (even with tenuto, etc) next to each other start a new note playback for the second note, no matter how "linked" the two notes are. This has a very bad effect for instruments that have the ability to change their pitch mid note, such as the human voice.

For instance, if you "Ahh" thirds, you don't typically go "ahhhAHHH" (starting a new "ahhh" for the second note) but instead go "ahhhHHH" - that is, changing the pitch of your existing vocalisation to the new note without starting a new vocalisation.

Some setting whereby an instrument can be tagged that "Slurs in this instrument should play as portamento" would be the ideal way to go - there would be no change to the notation or display, but the playback would sound a million times more natural. Especially if you can set a per-instrument portamento speed (for a voice you want it to be pretty fast - almost so fast you don't notice the slide at all).