First and foremost
JS Bach, at least long enough to rewrite all those pieces that have gone lost after his death (some 100 church cantatas, dozens of secular ones, tens of concertos, chamber works and whatnot and, of course, finish the Kunst der Fuge).
Monteverdi, to rewrite his lost operas.
Beethoven, who knows what he might have done with another 20 years or so.
Schubert, who knows what
he might have done with another
50 years or so.
Bruckner, to finish the 9th and, who knows, give us a 10th.
Mahler, at least enough time to write his 15th symphony.
Pettersson, a few more symphonies would be good.
Eliot Cartyr
I guess you mean Elliott Carter? By the look of the thing, he'll still be composing in another 20 years or so. Wouldn't mind that, actually!
Now how about composers who are, as such, dead but still composing? Such as Philip Glass?

Gep