The Roman have a very standarized religious system. Their religion is mostly performed at home, in day to day basis, to the homely Penates and the adoration to the ancestors. But there are also formaliced priest that presides the religious holydays. The most powerful are the College of Pontiffs, that consisted of the pontifex maximus and the other pontifices, the rex sacrorum, the fifteen flamens (dedicated to the early Roman panteon, like Jupiter, Quirinus and Mars), and the Vestals. The College of Pontiffs was one of the four major priestly colleges; originally their responsibility was limited to supervising both public and private sacrifices, but as time passed their responsibilities increased. The other colleges were the augures (who read omens), the quindecimviri sacris faciundis ("fifteen men who carry out the rites"), and the epulones (who set up feasts at festivals). During the republic, the intense comerce with the greeks merged the original panteon with the greek one, a phenomen named Interpretatio graeca. Other cults appeared in the late republic period and during the empire: Isis, Cybele, the mistery cults of Mithras and Sun Invictus, each with their informal priesthood and holy men. The roman empire was always ready to adopt the religions ideas of the all conquered peoples, being syncretized and merged. To create a Cleric in a Roman campaing, you can really pick any god or cult you want, roman or foreing. Some of the most interesting choices are the following. We have tried to select inspirations that are different to greek syncretized pantheon, that is mostly covered in Greek Clerics.
The goddess Isis was popularized in Rome after the conquest of Egypt. She was a patron of magic and wizards, so she was a appropiate fit for a cleric of the Arcana Domain.
Funeral rites in Rome were very complex. There are no really a "god of the Death", but the goddess Ceres (that is more known as a fertility goddess akin to greek Demeter) receives the ritual sacrifice to grant a pass to afterlife. So, a priest of Ceres can operate as a Grave Domain cleric. If the rites are not completed succesfuly, a dead one can return a Lemur, a generic term for many forms of undead. The deity that is chief of the underword and the king of the Lemures is the almost secret god named Orcus, so a follower of this cult can be cleric of the Death Domain.
One of the old gods from Rome, Consus, started as fertility god, but their priesthood instigated the Rapt of the Sabine Women, and since that he has become associated to the Trickery Domain. The god Mercury is also god fit, being a roman translation of the classic trickster greek god Hermes.
The solar god Sol was one of the very early gods of the roman, that was eventual merged into myths to Helios, the greek (minor) solar deity. But the cult increased in popularity later in the Empire, under Aurelius, with the name of Sol Invictus. A cleric of the Light Domain can be a follower of either of those versions of Sol.
Mars was an early god of the grains and the spears, one of the original gods of the Roman people during the Monarchy. Later, it was syncretized with Ares, and become the main patron god of War in the Rome cult. One of the high Flamens is the Flamen Martialis, and he has responsabilities with the armies, but due ritual tabooes he is unable to participate directly on the battle. You can assume that the Flamen Martiales has some priest that can actually form part of the legions, and those are the Cleric of the War Domain.
One of the ritual colleges of the Rome Republic are the Fetiales, a corporation of expert diplomats-priest that advice the Senate about foreing issues, and form part of the declaration of Peace and War. A cleric of the Peace Domain can be a Fetial.
The phrigian goddess Cybele started to be adored in Rome at the late republic period. It was a nurturing and protective goddess, know as the Magma Mater, and could fit the Protection Domain