I think the most compelling argument for the immorality of abortion grounds the fetus' right to life in its potentiality to develop into a person. A fetus, just like a newborn baby or child or a sleeping person, is not a person in act, does not actually exhibit personhood while in that state. At the same time we generally do not find the killing of infants or sleeping persons morally acceptable: their potential personhood is sufficient condition to make their killing tantamount to murder. Hence, etc. in the case of the fetus.
Here's a similar line of reasoning applied to the theological doctrine of universalism, according to which all persons will be saved. The argument goes like this: all human beings do not actually exhibit that glorified, deified state which persons will have in the eschaton, but nevertheless, in spite of their actual sinfulness and only potential glorification, it is immoral to kill them. Presumably, however, this would make it immoral for God eternally to punish or destroy a person whose salvation he could have guaranteed: it would be destroying that which has the potentiality to develop into something of great and tremendous value; it would be akin to killing a fetus when it is a potential person.
If someone denies that a potentiality to develop a certain trait or mode of being can ground an actual right to life, then another argument has to be given against abortion. It's not obvious that there is any good one which does not ultimately ground the fetus' right to life in its potential to become a person. More than that, such a denial is just not plausible: a person in a deep sleep is not exhibiting any of the characteristics of a person -- e.g., consciousness, the use of language, moral agency, etc. -- but it would not therefore be moral to kill her. Likewise potential works of art, such as drafts of novels or songs or poems, are of great value to us even if they are incomplete and imperfect, because in them exists the potentiality to become something of great price. They receive a sort of derivative actual value from the potential value of their realized state.
For this reason, short of denying that the persons who are going to be damned have the potential be glorified -- a ridiculous and implausible position if ever there were one, since it amounts to denying that they are even human -- it would seem equally immoral and inappropriate of God (at least to fail to do his part) to realize the salvation of every human person.