Difficult to say...but to take Mendelssohn as my example: If he would have been born into Baroque epoche, he would have been writing like Bach, but with tendency to Buxtehude, because Bach might have been too "revolutional" in his early years.
Mendelssohn was conservative in his days, and today he would be also... I think if he would be alive, he would develop polyphony into fields which weren't known yet but totally tonal. He would go on combining polyphony and homophony, he would try a Renaissance of works of a clear line but with high level, music of heart.