For sure aside to Saint Peter (the Constantinian Basilica of course) there is the so called Onorian Mausoleum, with the tombs of Honorious and Galla Placida, but at the time wasn't directly connected with the church. It likely touched the transept but there wasn't a direct access until the mausoleum wasn't converted in the 8th century in a church (Saint Petronilla) and then a direct access for Saint Peter was opened. So in principle Emperors could have built their tombs in the Vatican hill but if else Honorius's mausoleum is the proof an Emperor unlikely would have been buried in the basilica.
This is a late map of the Basilica. You can see here the Onorian Mausoleum connected with Saint Peter through a latter built atrium. But this design is interesting also for the various annexed rooms on the sides, essentially built in successive periods. Here is possible TTL certain Emperors would decide to build TTL those chapels where placing their own tombs, while embellishing the basilica, or because they imposed their will on the Pope of the time or else. But the case of Honorius's mausoleum is indicative of the fact the basilica wasn't designed to host Imperial tombs, only a certain one. At least, not in those times.
Honorius's case is interesting because he was the last Emperor effectively buried in Rome, maybe with the exception of Olybrius as the only one dying of natural death but is unclear where Ricimer buried him. So now we would have five Imperial mausoleums in the city. The ones of Augustus and Hadrian, surely not usable; The one of Constantine given instead to Helena; the one built for Constantia (Saint Agnes); and the Honorian one. Plus there is the Column of Trajan which is an exception.
But, that Marcianus would built a mausoleum out of the walls was not only an act of convenience but would also of tradition, building tombs out of the Pomerius line; and we can't forget the laws about burying out of city perimeters still stand.
Now, about possible and future burials inside Rome I can see those patterns:
A) An emperor would decide to bury himself or a relative in a church inside Rome or inside Saint Peter. Hence or he twists the arm of the Pope of the time, or reaches an agreement, causing a precedent for future Imperial burials.
B) Why abandon the practice to bury Emperors out of the walls? Possibilities 1) period of relative weakness of the Empire, with growing raids in the countryside (rebel armies, mercenaries, invasion forces, etc) against the mausoleums around the city. Pushing emperors of the time to bury inside Rome and the Pope caving it for state of necessity. 2) adoption of eastern Imperial costumes about burying inside churches in the WRE, through permissions and concessions to the law. Point would be if last word would be of the Church or the State, however. 3) economic distress of the Empire which would force an Emperor to build a more modest tomb inside a church of Rome.
C) About Saint Peter: technically burials in the basilica could be possible because out the pomerius line, but again Honorius's mausoleum shows the late Emperors won't see it as a proper place for their burials - aside yes, inside not. And besides technically as Pontefix Maximus a Patriarch could rejuvenate the Pomerius into a Christian declination and extend it to the Vatican, to stop an Emperor wishing to have a tomb in the basilica... just a supposition but still fascinating.
After this rant about the fact an Emperor of the time won't seek burial inside Rome, now we must see where Marcianus could have built the site. You said "half a day of march from Rome". That's quite the long distance because it implies at least 20 kilometers of distance from the Urbs... Hence spreading from a radius touching Ostia, Tivoli, the Lake of Bracciano, Monterotondo. Is quite the distance for an Imperial tomb IMO but for now let's keep this distance in consideration.
About the road. Nomentana or Salaria? Well there is Constantia's mausoleum so could be a good area for future Imperial burials unless Marcianus wants to build in a zone where his complex would stand isolated and emerge.
Ostiensis? There is the advantage being between Rome and Ostia, to be in a safe enough place. But there is the risk of floodings from the Tiber.
Appian? Maybe in a free spot around rests of other tombs and monuments and ruins along the way... also quite trafficked road, with benefits and not.
Flaminia or Aurelia? The main risk is they are the main connection with the North and where an enemy army could likely march towards Rome... plus are quite near to Saint Peter.
Personally I would be for Ostensies or Portuensis at least.
