Hmmm, I think that, with the addition of Culture Groups and Civ Characteristics, there IS a way you could bring back the senate-but in a better way than it was implemented in Civ2!
Firstly, if your civ is agricultural or scientific, then your senate might be LESS likely to agree to go to war, wheras militaristic and expansionist Civs would have much more HAWKISH senates. The other civ types would be roughly neutral, with their general character being determined by their other characteristic-e.g. an industrious/scientific civ would be less agressive than a militaristic/religious civ! Other factors which would influence senate interference in war/peace declarations would be:
1) Culture groups: more likely to accept peace with same culture, and more likely to accept war with different culture-and vice versa. So, for instance, a senate in a civ from a European culture group would be more willing to accept your offers of peace, alliances, RoP's and trades to England, say, than to the Chinese!
2) War Weariness: The higher the current war weariness, the better the chance of your senate pushing you to accept or offer peace!
3) Government type: If a civ has your civs preferred gov. type, then your senate will be more prepared to be peaceful towards them, and less likely to want a war-and vice versa. So, for instance, your Democratic, German Senate will probably try to oppose any peaceful overtures you might make towards the 'evil, communist Persians'

!!
4) Having the UN: Like in Civ2, you should have a better chance of declaring war if you have built the UN! By the same token, you should have less of a chance of declaring war successfully AGAINST a nation that has the UN!
The senate should also be able to effect other diplomatic agreements, based on the same sort of criteria! For instance, senates from commercial Civs might try to push you into trade deals for resources, gold or luxuries, wheras Expansionist and militaristic civs might, aside from wars, try to push you to sign MPP's, ROP's and Alliances.
Anyway, thats just one thought, based on the current Civ3 model. Another, more radical suggestion is to have some kind of 'absolutity factor' and 'Influence' system. In this system, each segment of your society-from workers to the military to the Wealthy Elite-have a certain degree of influence on your civ. The amount of influence is determined by civ characteristics, current religion and laws and the number of types of improvements you build. The higher the influence, the more chance that group has of forcing your hand on domestic and foreign policy issues.
Base influence is modified, up or down, according to your governments 'Absolutity factor' or AF-the measure of how much Absolute Rule you have! Negative factors increase your control, wheras positive factors give more power to your citizens! Within this system, certain government types would also allow you to build Small Wonders like 'Parliament' or 'Senate'-which improve happiness and culture, whilst reducing corruption and War Weariness. However, they also increase influence of your civs citizens on your decisions!
Anyway either way, in my opinion, would be an acceptable way to bring back the Senate-in a fashion which would be FAR less frustrating that the Civ2 model!
Yours,
Aussie_Lurker.