I think you'd be able to have this with a PoD that tied Caesar to the Danube. Danube means his ambitions are in Pannonia and the Carpathian Basin (and the Getae region), areas he intended to campaign in IOTL.
That creates a different frontier to Gaul, with the Danube being the heart of a NW movement into Germania, and the need to fortify the Carpathian passes.
No Caesar in Britain, or Gaul changes that up a good bit, but by my understanding, the Carpathian basin is more of a settlement effort than Gaul would be, and as such less wealthy.
But that changes our story only a little. Expanding into Noricum and potentially OTL Czechia makes sense from that perspective, if the Romans learn anything about it, but it'd certainly be a case of strengthening the frontiers that had been captured, and its still an impressive land-grab.
I can't help but think that it is a Poor Mans Gaul. Potentially a springboard for THIS Caesar to play politics to invade Gaul outright, but I can't fathom if it'd be better or worse for him. It might instead mean that he adds a fourth to the alliance, who would be the guy to conquer Gaul, or complete Illyria whilst Caesar uses his Illyrian successes to relocate to Gaul. Perhaps even Titus Labienus could be a player there. Whether that means we have a Pro-Senate Illyrian force that comes to bite Caesar, who knows.