Seleucids (in fact all the Hellenistic kingdoms) were on a long slow decline post the Successor wars. The odd good Emperor halted the decline and even reversed it but my money would still be on the Parthians reaching the Mediterranean
 
I have no real idea, it depends from a lot of contingent events. Personally, I'd say that Celtic Gaul is probably on its way to provide Arverni with an institutionalized dominance (sort of high-kingship) at term if they manage to clean their political orbit. For my TL, I'd assume something like Ist century CE, but that's a vastly different geopolitical situation.
High kingship over the territory delineated in your first map in the TL?

As for the larger scale...It's not known if Celts considered themselves as entierely related, altough the notion possibly was there regionally (Germani is a translated term for a Gaulish word for their neighbours, underlining their kinship), and exchanges between Brittons and Gauls certainly raised some acknowledgement. Now...they didnt considered theselves the same, that's probable. But even with distance and time, several basic features remained (Galatian language was enough kept together in the Vth that it apparently was similar to the Gaulish speech of Trier)
Well it kinda goes without saying, but similarities tend to be noted very differently, same goes with perceived intelligibility.
It's hard to really even closely pin down how similar those 2 were, although I imagine by analogy their relatedness would certainly remain evident with "just" 6-7 centuries of distance.

You'd still have a Germanic advance southwards IMO, while less unidirectional again, but possibly more mixed-up as a result than IOTL (Germanic campaigns and Batavii wars did their job too on this regard) with dominance of Celto-Germanic mix up in the Rhineland IMO.
What about the Upper Danube basin and Pannonia? Also would oppida still remain prominent there with a Germanic influx or just passage of time?

In fact, for what matter right-bank Rhineland's peoples names (roughly up to Weser), here's a quick summary of names that could be traced from Celtic or Germanic roots

Chaucii : likely Germanic (although a Celtic cognate might be possible)
Frisii : likely Germanic
Batavi : possibly Celtic or Germanic
Cannifeates : possibly a Celtic/Germanic mix meaning "masters of garlic/leek"
Chamavii : likely Germanic
Bructeri : unknown but some names have a possible Celtic origin as Veleda.
Dulgubnii : likely Germanic
Cherusci : likely Germanic
Ubii : possibly Celtic or Germanic
Sugambri/Sicambri : unknown, but their kings Baetorix and Deudorix (Teutorix?) had Gaulish names
Cugerni : likely Celtic
Marsi : likely Germanic
Chatti : likely Germanic, as well related groups among the listed peoples
Tencteri : probably Celtic as their name can be translated from Gaulish as "Faithful ones", (there's Germanic cognates, but that's the case for a lot of words possibly borrowed from Celtic)
Usipetes : probably Celtic as their name can be translated from Gaulish as "Good Riders".
Tubantes : maybe Celtic due to relative stability of settlement, but the name can be explained trough Germanic and influence from Chatti
Mattiaci : maybe Celtic due to relative stability of settlement, but the name can be explained trough Germanic and influence from Chatti
Quadi : unknown : probably the same than Marcomanni and Suebi up to a point.
Vangiones : likely Celtic (a relation with gienno?)
Nemetes : likely Celtic, from nemeto
Marcomanni : likely Germanic but their king Marobod had a Gaulish name, and ethnonym could be explained trough Gaulish as well (without being really convincing) as "Riding people".
Suebi : likely Germanic, but their king Ariovist had a Gaulish name and the tribal name can be explained trough Gaulish as well as "Wanderers"
Latobrigi : likely Celtic
Tulingi : unknown but probably Celtic
Triboci : unknown but some names are probably Celtic

It's definitely not exhaustive (or entirely accurate : please correct me if you spot anything wrong) and does not pretends to display any linguistical, and even less ethnic characterization, but to point that the Celtic element was still pretty strong even in the Ist century CE even if it was replaced by Germanic elements more and more significantly.
The difficulty in differentiating Celts from German in the region should make us more cautious in drawing a line, and rather consider the Rhineland region in the Ist century BCE to be more of a march whom peoples were close enough from what existed in Gaul.
Note that the southern you get, the "Celtier" it is.
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I'm wondering, how plausible is the idea that various rulers(Ariovistus for example) had more than one name or nickname for various linguistic group that said ruler dealt with? For example "Attila" is possibly a Gothic nickname.

Also prior to Rome's intervention, what territories did this intermixed region compromise? How far did Celtic influence go northwards and how far did Germanic?
Germania being the quintessential "borderless" region, I'm not sure how it would be considered long term. Maybe as a march in its western areas? I'd go with that myself, but that's entierely speculative with such an important PoD.
A march between/of what?
 
so, instead of those, maybe a Gaulish version of Genghis Khan or Atilla? One particularly brilliant warlord who comes to dominate first his own people and then the neighbors?
That's possible, altough both of these leaders (for different reasons) existence and successes are tied to the own existence of a neighbouring big imperial state sitting on the economic and political centers of their world.
Something like Norman conquests of Italy or England, or even attempted grabbing at Anatolia (and generally following what Gauls did IOTL) seems to me a bit more likely in a forseeable future. Or following ancient comparisons, more Barcid-like in building up local hegemonies.
 
High kingship over the territory delineated in your first map in the TL?
On territories but as well their populations : we know that they were instances of peeoples essentially forming true federations (Remi and Suessiones, for instances) and other (like Menapii or Arecomici) splitting over the main people territory to have their own "city", with black jack and hookers.

What about the Upper Danube basin and Pannonia? Also would oppida still remain prominent there with a Germanic influx or just passage of time?
Frankly : it's anyone's guess and it depens on what happen on the Pontic Steppe and Balkans. I'd tend to think Raetia and Bohemia have fair chances to have a distinct Celtic background with a more or less important Germanic part, but that's me.

I'm wondering, how plausible is the idea that various rulers(Ariovistus for example) had more than one name or nickname for various linguistic group that said ruler dealt with? For example "Attila" is possibly a Gothic nickname.
Or it's, simply, their names. I agree that names aren't by themselves proof of cultural background (my own name have an Hebrew origin and I just don't think I've an Israeli background) but it certainly highlights some cultural presence (and for a king/leader, it's far than irrelevant). Regarding Huns, consiering that Attila, his predecessors (Bleda and Ruga) or his son Ellac were probably Germanic or Germanized names, it might not be an oversight of litterally every contemprary scholars but his actual name (or at the very least its usual name).
There's some circular reasoning thinking that, because he's an Hun, his real name must have been Hunnic (even if we couldn't recognize Hunnic anguage if he jumped all over us and trampled our head asking for tribute), and he must have been Hunnic and not Germanic because Attilla was a Hun.

The same goes for Arviosist and the whole of names rather well explainable trough Gaulish : that they appear regularily does points to a likely mixed cultural bag in the upper layers of these peoples at least.

Also prior to Rome's intervention, what territories did this intermixed region compromise? How far did Celtic influence go northwards and how far did Germanic?
It's really hard to say, because without written evidence we have to look at material culture : and then we're reminded that several Celtic features are often regional and their absence doesn't mean they were not related. No oppidae doesn't necessarily mean it wasn't Celt, just that it wasn't similar to this Celtic culture.
Conversely, what does artifacts like this tell us? That Celtic features were present as far as Danemark? That Germans and Celts of Germania exchanged goods and practices?

While we can define the extent of Celtic culture trough Halstattian and Latenian material culture evidence, it's not this much decisive in making the difference between Celts and Germans (possibly more or less importantly Celtized, but also dynamic enough to advance south). Similarily, we're able to find the rough limits of Iron Age Scandinavian/Geranic culture, but there's a whole region in between that it's hard to attribute after the IInd century BCE.

So, its really only safe to depict Middle-Germany as a mixed area after the IInd century BCE, and before that possibly Celtic as long material culture is concerned.

A march between/of what?
A march isn't necessarily a military march, but also a buffer region, a border zone, etc. In this sense, a set of peoples influenced by their neighbours, not exactly part of their neighbour ensemble and essentially defined by being the "Last Chance Saloon" of local geopolitics.
 
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