Be aware: Long post about a game mechanic that mostly affects highsec! 
TL;DR at the end - anyway, here we go:
It’s broken!
Wardec mechanic is broken. Despite driving a lot of new players out of the game, the wardec mechanic in it’s current state does not lead to interesting gameplay. It rather makes people not to undock (or even log in) instead of encouraging people to fight - which should be the real goal of a war(dec), right?
A goal
So the most important part of a good and healthy wardec system is already found: You have to fight for/about something. A war needs to have a target. Instead of just being the button that disables CONCORD, it should be the tool to reach an aim. May it be to destroy a structure or to push those miners out of “your system” or just to take revenge on someone; you need to set a goal which will define the wardec.
Win or lose
This also helps with the next thing a good mechanic needs to fullfill. And that’s recognizing a clear winner and loser. In the current state, the war ends after 7days (if no one renews it) and that’s it. Not even a “congratulation” is given out, as the game does not know or care who won the war. But this is actually a really big deal in the human understanding of war. The game defines an attacker and defender, but doesn’t follow up if the attacker can achieve what he is after (since there is no target atm) or if the defender could avert the enemy from reaching the goal.
If we take a look to Nullsec wars, where wardecs aren’t used, we can clearly see who won a war (with a very few exceptions) by looking at the objectives and who took them. This is usually also used when looking at a single battle, while the always present ISK-war gets added or is the main focus. In the latter case, the ISK-war becomes the objective, which is absolutely legit of course.
An example
So summarizing those points, the new wardec could look like that:
The attacker want’s to start a war. He chooses a victim and a goal. A citadel for example, or simply an amount of ISK they want to have destroyed by the end of the war. The goal should also have an influence on the length of the war, but in general the attacker should be able to select how long the war will go - in a certain timeframe of course. This could reach from a 3day war to destroy 1bil in ships up to a 10day war to destroy a structure for example. The defenders have the same goals. If it is a citadel, they need to protect it. If it is an ISK amount, they can reach it before the enemy. You don’t really win a war if you shoot 1b, but lose 5b to do that. As always the numbers are just examples and I would trust CCP to finetune those to a healthy state. Yes, I really do. 
Regional limitations
That brings me to a second part of the wardec mechanic. It wouldn’t make much sense to fight over an Astrahus and be allowed to shoot the defender in all of New Eden. And if the war is to drive someone away from where you wanna live in Highsec, a wardec that doesn’t “show a direction to flee to” is not very helpful either. So the new wardec system absolutely needs regional restrictions. It should be possible to select the region(s) the war should cover, that should of course also have an influence on the cost of the war, and it needs to be chained to the selected goal for obvious reasons. The attacker should be able to select a system, a constellation or a complete region. I am not sure if several regions should be possible to choose, but I would not expand this to more than the borders of a single empire.
With this, you allow people to actually run from a war - which is purpose of the design. If an attacker drives someone out of a system/constellation/region, they should have gotten what they wanted in the first place, as the area is free from the targets and the goal is reached (or reachable).
Actual fights
But since the mechanic should encourage fighting (and not just flying from one area to another), how do you get people to fight if they don’t really feel the desire to do so? Well, that is also possible: You make them lose something if they don’t come to the fight AND give them a reward for winning.
In case of a structure as war goal, it’s simple: It get’s blown up. In case of the “system drive out” it’s more complicated. But you could make war-losers criminals (also to the local law-enforcement) for the same time the war was ongoing. (10 days of war = 10 days after the war you are not allowed to enter the system). This would be a mayor drawback if you lose a war.
On the side of the winner, you should get additional rewards other than just reaching the goal. Maybe half of the money the war has cost? Or a pre-defined amount of some sort. This part is especially important in the case the defender wins - he would otherwise have nothing to win. Here is much room for CCP to play around and also PLEX or SKINs or whatever can be given out. Of course related to the war-goals and duration etc.
One thing that has been brought up as “fix” many times are simple restrictions in the amount of wardecs you can get or the time you can get them. I don’t like those but would highly suggest to restrict the zones someone can get wardecs in. By either allowing only 1 region/empire to be wardec’ed in, or by allowing all but 1. This way you are always able to play (although you might need to evacuate).
TL;DR // summary
There needs to be a goal (structure/ISK/space) set by the attacker.
A region needs to be chosen by the attacker.
And a reward for the winner (this can also be the defender).
Then the Wardec system is fixed and everyone’s happy.
Now feel free to discuss 
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