The problem was that after taking Rome, the allies turned among themselves like dogs fighting over the same bone - the loot from Rome, Brennius despite his genius lost out. If Brennius could hold the tribes together, he could easily hold onto Gaul and maybe part of Britannia, but I give the Romans and Carthage to rethink their strategy concerning this nation - they would play this nation like a toy where they would have it fight their war for them, tear it apart province by province, or in the case of the Romans stage a revenge war. The other option is that Brennius decides to move in Rome where-in it becomes a Celtic / Roman Empire under the leadership of his descendants.