In many European languages - at least those I looked into - it is possible to use Present Subjunctive as optative mood (although often considered dated or of limited availability):
Thy Kingdom come
Dein Königreich komme
Que ton règne vienne
PIE is supposed to have had an optative and a subjunctive mood. As Latin evolved, subjunctive and optative mood merged. Proto-Germanic however, dropped subjunctive mood and optative mood became subjunctive. I wonder if Germanic languages never dropped optative mood entirely or if it was reintroduced from Romance languages/Latin at a later point; if therefore there is an unbroken ancestry between present day "subjunctive as optative mood" and PIE optative mood in Germanic languages or not.