I tend to play a mini roleplay with my game and so I at least want to find a cassus-belli for invading someone. In my brand new Casse game I have moved over and am taking out the sweboz from the north. The land is dirt poor, the troops are difficult to beat because they have built up with about 12 towns and frankly I don't think I can hold my gains. BUT they are at war with both my allies (aedui and averni) and were starting to sucessfully take towns in gaul.

It all depends on how you play but I like to justify what I am doing and have a bit of fun with the game inside the game.

Alternatively, you should be able to recruit decent troops in Gaul itself so 'unifying' the celts and taking gaul is a decent goal that is also good gameplay sense. And Spain/Iberia is wealthy as hell with mines and decent access to other factions for sea trade so if you wanted more cash then thats a worthwhile goal gameplay-wise. You can also 'liberate' the celto-iberians if you care about them.