I doubt you'd have a maintained unified imperium : at this point, it was largely accepted that it had to be shared, particularily between West and East.
Remember that Theodosius' unification was more or less accidental, and in any case short-lived : no more than 4 months.
Giving the growingly more important military function of the imperialship, a division will be likely to happen ITTL as well, even if Arcadius is the sole heir of Theodosius (I'd expect him to be Emperor in the East, as he was already junior EREmperor before Theodosius' death IOTL).
Who would be in charge of the west is anybody's guess : I'd think, tough, that some people as Nebridius (which is largely unknown, safe he was a nephew by alliance of Theodosius) or the childs of Thermantia may have a better chance in spite of their relation with Magnus Maximus.
Eventually, if Pulcheria survives childhood, her spouse will be well placed.