[Gameplay] After destroying a few factions the others should team up against me.

Agreed, Warlords used to do this.  When it's obvious you're going to conquer the world, the others should just team up against public enemy #1.  They can resume their squabble after they've put you down, but of course for you the game would be over so you wouldn't get to see it.

Well, it depends.  If you've just destroyed the bitter enemy of another faction, that faction should love you -- moreso if they were losing their war with that enemy you just destroyed, and yet even moreso if the one you destroyed was on 'opposite' side of kingdom/empire.

You should definitely lose favor with all other civs if you destroy someone you were supposedly on the same side with.

Come to think of it, there should be 'mercy' options, allow an enemy sov to live as your subject, paying you royalties in gildar and perhaps other resources in exchange for you sparing their lives.  Might be neat to have tactical mercy options as well, sparing the lives of enemy heroes if they join your side.

I think a diplomatic modifier would be a good way to get the desired effect. I doesn't matter if the AI you are destroying is a friend or a foe with the other AI's. As soon as other nations witness how you completely obliterate another realm and its sovereign they witness your warlike nature and become more wary. This could be a modifier to the already existing diplomatic modifier about army strength. A large army of a peaceful country is far less scary than a large army of a sovereign who has already proven to the world that he/she will use it to destroy and conquer.

That's utter hogwash, frankly.  I wiped out an AI that declared war on me, and my reward for survival is suddenly everyone wars with me?

First war is not a crush or be crushed affair. War can be stopped after some cities have been (re)conquered or you let the enemy live on with only a small realm. Second it should not be the case that everyone else goes to war with you immediately but that they are wary of your actions from a diplomatic point of view. In Master of Orion and other 4X games for example the other races are more inclined to gang up on you after you use biological weapons. In Elemental it should be similar with exterminating a sovereign.

There should be several Modfires -   One If you had war decleared on you.  This would Lower the damage done if you defeted the kingdom/empire that went and declared war on you.   

Now if you declare war, and attack the same turn you take a negitive diplomatic  hit.  Your underhanded tactics have disgusted every one else. 

Say there is a large negitve for attacking the same turn war is declared,

Each turn you wait up till 10 before you attack, you will suffer a much lower negitve veiw  And after 10 turns the others veiw that your kindom/empire is acting honorably by letting the other side prepare.    This is only for the side that declares war...  

Each Time you defeat a faction every other faction is more and more warry of you...

If you are clearly overuning the world the others should band togater to confront you...    Now I would still like to see to see some back stabing going on.. if two sides were at war before every one woke up to the fact that they are all next on the menu...  

Which part of "A FEW FACTIONS" is not clear in the title of this thread?

You can win a war and that's OK but if you start to methodically destroy anyone you meet, that's a bad sign from any point of view, besides that is coherent with what would happen if you played against human opponents: If you have an expansionistic attitude the other human players will certainly attack you before it is too late, or they would just do it because they don't want you to win!

But with your kind of thinking you don't improve the AI: you expect the AI to be realistic when in fact we, as player are not. We just want to win a game.

In current system, if you have overwhelming army and treaties with AIs they will adore you. So diplomatic victory gets very easy to get. Way to easy.

I would say it depends more on how many other factions were killed.  If I was the elves at war with Saruman and I witnessed Sauron kill the race of men and then kill the dwarves and then kill Saruman then obviously I wouldn't be so happy to see Sauron.  Any race which has killed 3 or more factions should be viewed as a threat unless you have allied with them.

Besides, if the whole point is winning, the AI cannot pursue a diplomatic victory because me as a player won't accept to be the last one to ally to an AI faction thus making it win, so why should they allow me to have a diplomatic victory? maybe I am missing something about the way diplomatic victory works, but it seems a bit weird every time the AI is forced to act realistically while the player just want to win a game.

I tend to agree that diplomatic victory needs to be reworked.  All you need is one hold-out and it's impossible to do, and really the AI should be holding out if it knows it will just be handing you the win.

I've never seen Diplomatic victory done well in any game.  It almost always boils down to a vote.  If anyone gets enough votes to win, then everyone else dies.  Why would anyone EVER vote for anyone but themselves?  Anyone who does vote for someone else is essentially committing suicide and trying to take almost everyone else with them.  In essence, it's a spiteful defeat condition that leaves one man standing, not "diplomatic" or a "victory".  In our case, we vote by making alliances, so there is some reason to vote for someone else (problem 1 solved).  The problem is, anyone who makes the last vote is essentially committing suicide and taking everyone else down with them (problem 2 is as bad as ever). 

I think perhaps the best mode of diplomatic victory would be to allow for diplomatic annexations of rivals, and then leaving this as a subset of the "conquest" victory type.  This would still mean you'd have to be careful of who you make alliances with, but it wouldn't make this quite so spontaneous or artificial.

As to the main topic, I think more to the point is that weaker factions should be coming to each other's aid if they're attacked unprovoked.  Don't wait until after the battle is over and the superpower faction is getting ready to declare war against you anyways.  Jump in and help your weak allies while they're still around to help you beat the stronger faction!

When a faction is significantly weaker that at least one other, it should be more willing to have an alliance with anyone, not just the "super powers". And more actions need to affect diplomatic stadings.