"Gaul" wasn't an enemy of Rome though, it didn't exist except as a geographical region. About a third of Gaul was already under either cultural and economic control of Rome, or else directly ruled by it through governors. There was no implacable enemy, the days of Brennus were 300 years past and while there was a certain prestige to it, it was not much compared to the East or Egypt, I think.
The Cimbri and Teutones that so terrified Rome for a decade most likely came from what is now southern Denmark, not Gaul.
Caesar's reasons for going to Illyria and Dacia are exactly the same as those for going to Gaul...except that Illyria and Dacia are actually wealthier than Gaul and the Scordisci and others there had been fighting a running raid-and-guerrila war against Rome there for over a century. A much more solid and implacable foe than the docile, Romanized Gauls in southern Gaul.
That's both severely overselling the Scordicians and the Dacians and underestimating the Gaul. By Cesar's days the Scordisis where far from their glory days, in fact those where gone since about a century before that when they had been beated to the pulp by Scipio Asiaticus several times, their raids had grown far more hesitant after that. The Dacian had an opposite problem, they where in a good place under Burebistas but even that must not be exagerated as Cesar essentially considered a second-rate army with secondary commanders in command enough to submit them while he was playing the big game against the Persians.
The Gauls, on the other hand, took around ten years to be forced to submit by Cesar himself and many of the suposedly docile tribes closest to the Gallia Narbonensis played in fact crucial roles in Vercingetorix rising at the end.
Besside, you are approaching this from the wrong angle: propaganda value came from the emotional impact of the conquest of Gaul. Sure, Roman traders did went around and the south of the region was becoming more and more latinised but that didn't change the past and the place the Sack of Brennos still had in roman imaginations. The mark left by this traumatic even ensured that Gaul was far more emotionally important then the Dacians and the Scordisis could ever be.
Morever, unlike all the other ennemies of the republic, from the Etruscans to Mithrades, the story of the sack of Rome couldn't be written in a way that truly showed the roman as complete victors, having ennacted brutal and disproportional retributions for past wrongs. Cesar seemingly changed that. Cesar was looking for a campaign that would instantly mark him as the greatest roman alive in the eyes of most, something with the utmost amount of glamour, Dacia and the Danube just didn't fit the bill as well as Gaul and why he let circumstances engage further and further into gallic affairs.
The Cimbric and the Teutons didn't come from Gaul but their campaigns showed how much of a blind spot an independant Gaul was for Rome as its essentially allowed an ennemy to get at Rome doorstep before encountering any legions, a frontier at the Rhine was far more secure as demonstrated again by Ariovistus operations in Gaul.
Cesar did plan for a campaign in the Balkans at first but as soon as events gave him an opening in Gaul he forgot about it in an instant and I don't see him as the type of leader who would let himself be entangled in a situation if he didn't want too. If he decided to push deep into Gaul after dealing with the Helvetis it was because a campaign offered, for both him and Rome, benifices that couldn't be found in the Balkans.