Oh there are so many! To take just composers to start with:
Mozart and Schubert. Given an extra five years what might they not have achieved? And Schubert would still only have been 36!
Also Mendelssohn, together with his beloved sister, Fanny Hensel, an interesting composer in her own right, still too much neglected.
Later in that century, Tchaikovsky. In five years we could expect perhaps another symphony, another opera and another ballet? Bizet - he deserved to live to experiance the phenomenal popularity of Carmen, which he never knew about, and would probably have written at least one more opera.
A couple of 'might-have-beens': Juan Crisostomo Arriaga, the 'Spanish Mozart', dead at 20, and Thomas Linley the younger, the 'English Mozart', drowned at 22. Give them both another 10, enough time to achieve greatness.
Henry Purcell should certainly have had at least another five years - what else might he have achieved? And perhaps Pergolesi. Oh, the list is almost endless!