One alternative (to avoid the creation of an extra office) would be to make some other office holder - chief justice, ppt, something like that - the official head of state as a secondary function. Works only if the head of state has
no powers, though. (For example - the German President can refuse to sign legislation into law, but only when he is convinced that it is unconstitutional - doubts about its constitutionality are not sufficient
. Even in that case he can be overridden by parliament. That's pretty much the only power he has, except when parliament is having problems finding a government; however even that is not a power I'd want a president who's also the CJ to have, obviously.)
On second thought - wouldn't the VP be abolished? A President and a Prime Minister isn't really more positions than a President and a Vice President, is it?