Admittedly, I know very little about this, but is it possible for the Arian interpretation of Christianity to triumph (how?) with Nicene Christianity being regarded as heresy in this scenario.
Eventually, the triumph of any trinitarian conceptions in the IVth century (and onward) depends on imperial favour, maybe more than strict theological controversy. By the time Ancyre/Nicea is organized, Arius is too isolated clerically to really hope being considered worth following as such.
You'd either need an earlier Arian-like equivalent to appear in the II or IIIrd century , along say Marcionism, to really have a chance at being established as a strong current of the "main" Christianity and then being a viable alternative to maintaining the religious unity.
What you could get eventually, as it happened IOTL with the late reign of Constantine but especially with Constans II and Valens, would be the promotion of a middle-way to resolve christology issues : subordinationism was sort of a semi-arianism on some regards (o the point its opponents called it Arianism to discredite it) and it had some success both among Romans and Barbarians (who converted first to this imperially-sanctionned belief. But while it could last ITTL instead of Niceanism, it wouldn't be Arianism and probably wouldn't consider itself as such.
WRE still falls, Arian Germanic tribes gradually convert their Roman subjects in former WRE provinces. Could we have Arian West and Nicaean East in such situation?
It's a common, but problematic, misconception to identify Arianism to Acacianism/Homeanism : apart an accusation of Arianism (in the same way Orthodoxy tended to define heresies with other ones in order to accuse them, such as frequent accusation of Manicheism way within medieval ara), there were essential difference.
First, while Arius stressed the non-divinity of logos and Son, Subordinationism grant them a subordinate divinity to the Father. Then, not only Homoeianism take great care not to detail and precise the relation within Trinity but frowned upon doing so because it was seen as dwelling into matters Men couldn't understand much and blaspheme a lot.
Originally, people like Goths converted to this branch of Christianity, paying lip-service to the emperor. Then while imperial religious policies changed, they kept it as a convenient identitarian marker to distinguish increasingly undistinguishable Romans and Barbarians.
In the West, peoples as Burgundians and Suevi first converted to a Nicean credo before switching to Homoeanism for the same reasons.
Now, let's imagine that western Romania remains largely non-christianized by the Vth century : it would ask for an early IVth PoD with no conversion of Constantine. It's perfectly doable but would have important consequences, such as no formal distinction between anti-trinitarianism, subordinationism and strict trinitarism. A bad way to start the whole thing.
Furthermore (and possibly ignoring the first problem), as an independent western Romania remaining pagan meaning absolutely a set of pagan emperors, Barbarians settling in the west would have little to no incitative converting to Christianity (Homoeian or Homouiousan).Not only Franks, but also Suevi, Burgundians, possibly Alans and Vandals, etc. Eventually, "only" Barbarians and Foedi converted to the Christianity favoured in the East (assuming that EREmperors made NO tentative to christianize the heck out of their western colleagues as they did IOTL when it was necessary) would rise as Christianised peoples other than Romans : either they quickly switch back to Constantinople's own religious policy; either they will more or less isolated religiously.
Note that even if western Romania remains pagan, at the moment you have an association between imperium and Christianity,at least a large part of Roman senatorial and landowning elites would be christianized (as their clients and surrounding populations) in places such as Italy or Africa in connection with Constantinople.
It's not that you don't have any possibility for subordinationist teaching to make their way in the West, but probably more as a limited influence with a Vth century PoD : Alaric II seems to have attempted a conciliatory religious policy in his kingdom before Clovis' conquest, by having sort of compromise between a largely Nicean population and some Homean (which in itself were already a compromise) beliefs.