I've said before that I think "Grand Duke" is a misleading title used
in English, as is "Grand Prince" (though less so). English has a need
for some word that represents about the same semantic area as Finnish
<ruhtinas> (from PG *druhtinaz, leader of a war-band), Swedish
<furste> and German <Fürst> (cognate with English "first"). So I asked
for some suggestions for a neoreflex for *druhtinaz in English, and as
it turns out, my own guess ("dright") was pretty close and that there
was a word for it up to the middle-ages: "drighten".
Let's say this word is re-adopted into English to mean a sovereign of
a state who isn't a king, or emperor. Male "Drighten", female
"Drighteness" (have to think of this one), and the appropriate word
for "area of rule" seems to be Drightendom. Adjective drightenly.
Possibly I will simply use "Principality" for 'Drightendom'.
Thus Finland is a Grand Principality ruled by Her Royal Highness the
Drighteness Alexandra II. Drightenne? Or possibly simply Drighten
Alexandra II.