Where should I go to next?

[Taedius]

I would focus on economy as well. What difficulty are you playing on? I would expect the Arverni to attack in the not-so-distant future, thereby giving you plenty of reason to counterattack and invade. Atleast you could make money from a punitive expedition. Then, of course, you'd get the attention of the Aedui. Then the Sweboz. All while the Lusotannan close in from the south.

As for RP casus belli, a strategy I sometimes use is to train a general of the culture in question, sponsor him with an army of his own people, and let him go in search for a kingdom on my expense. It is an expensive trick, but pays of in the long run. You could use it for Burdigala, or even Numantia. But yes, costly...