Both of you bring up some fascinating alternatives, one where the the Romans stop at Asia, and another where, instead, they forego deep expansion north of the Mediteranean shoreline.
One where an understanding is reached with the Seleucids, that their sphere of influence is on the Asian side of the Aegean, the Romans on the European. This would be Syrian War.
It would be interesting to see this develop and how it could come about. The Seleucids never allying with Pontus? Or the Romans making successful overtures to the Seleucids to stab Pontus in the back once Pontus shows an interest in Greece?
A water border for the Romans is much easier for the Romans to defend than the historic borderlands with the Parthians and Persians.
I would presume the Romans would eventually want Egypt for the grain and might have a land border with the Seleucids in Sinai or Palestine, or the Maccabees as a buffer state.
This scenario would likely see the Seleucids center of gravity shift westward, because I believe they were already losing Persia itself to the Parthians. So the core of their kingdom would now be Syria and Asia Minor, with Mesopotamia and Armenia being borderlands they fight over with the Parthians.
As for the Romans, they have shorter easily and cheaply defensible borders with Seleucids. That saves them from the costs of conflict to the east and allows them to concentrate their defensive strength on the European frontiers.
However, Asia Minor and the Levant were rather rich and productive provinces, so Rome is weakened by not having their revenues.
In the alternative,
In Europe: no Roman general conquers entire Gaul. Until Caesar, I don't think any general seriously considered this vast enterprise, and the Republic as whole was fairly content with the prosperous stretch of land connecting Hispania and Italia while maintaining cordial relations with some regional Gallic principalities and Massalia (which wasn't annexed until the Caesarian Civil war, if I recall correctly).
Rome is spared fighting and occupation costs in northern Europe (although over the centuries, Gallic and Teutonic threats to Italy and the Roman Mediterranean can re-emerge (as they had been threats at times up until the time of Marius). But it should be cheaper than OTL most of the time and make the cost of fighting the Parthians and then Persians more bearable.
In time, Rome has the opportunity cost of not getting the produce of Gaul (which was more productive than Italy for the 2nd through 5th centuries and don't get the mineral wealth of Britain's tin or Dacia's gold. An interesting alternative world, where Rome is basically entirely focused on what had been the historic empires (Carthaginian, Macedonian, Greek, Egyptian, Babylonian, Assyrian, Persian) of the Med and West Asia, where northern European exercises "self-determination" and is not subordinated to any Mediterranean state.