If there was a possibility for 'faction specific' type senate missions, that'd be nice. Gauls taking missions from the Carnutes and such, but it'd have to be depicted properly too. The druid-types of the Celtic peoples were not political leaders, they were advisors and religious leaders. The second position does provide them power in the aforementioned 'crusade' aspect. If you didn't perform the holy tasks required of you, the religious leaders could, essentially, perform a smear campaign of your rule, and encourage civil rebellion, desertion, etc., and their envoys to their peers in other Celtic nations could easily encourage their peers to engage in a similar campaign, and encourage invasion and open war against what would be seen as a heretical leader. But, that's all hypothetical, imagining that it COULD be done.

I only expand on the Celt suggestion here because I know essentially nothing about what could be done with other factions, but I imagine it'd be essentially identical. Most of it, for Celts, really lies in the wording of it, and the finer details exactly of what their 'missions' mean, and what their nature would be (religious conquests, not unlike crusades).

I know Pyscho V said all that, essentially, but I wished to expand upon it a bit, with the 'rationalizing' of it, because propaganda against you for defiance of a holy objective is probably the most realistic course of a 'senate' in a Celtic society, as they put a huge amount of stock in religious leaders, which, as stated, were not political in nature, but a religious leader, properly angling his influence to get what he desires, could become even more powerful, especially in a society that nearly deifies them.

My complaints with a 'druid' senate, that would govern ALL Celtic factions has already been noted, but I'd take no mind to individual 'senates', and special relationships between Celts, as Celts did take to mind who was Celtic and who was not, even if they did not consider themselves a single people, necessarily. And while their gods and practices may have varied some, they would all have a vested interest in ensuring one another follows their religion, because, seemingly, their religions likely encouraged a type of xenophobia, and encouraged some level of Celtic solidarity. This is most readily present in early dark age records of champion fights, where Celtic peoples (mainly Welsh and Gaelic by this time) would send only a single champion to fight another champion to the death, rather than engage in full scale wars, if they could be avoided. This type of mentality was still present after Christian conversion, where Irish kings tended to prefer this method of combat amongst eachother, and the one to use massed force first in a war was readily shunned and often allied against, not so much in support of the offended's argument, but to punish the offender for breaking a code of etiquette. As such, if ever possibly implemented (a one in a million chance, I'm sure), starting war with another Celtic nation should likely cause a large amount of revolt in their own country (a king may be feeling particularly irreligious or self important, but his people aren't necessarily so inclined to agree with them).