Suggesting 2 fundamental ideas for making Con builds viable.

All the other abilities have attacks associated with them, so, people use them because attacking is fun, soaking damage, not so much. Additionally, more AC is much more effective than more HP.

So, if there were attacks associated with con, then people would consider making con builds. There are plenty of ideas from other games, but, there is much that can be re-purposed in NWN2 and D&D also.

That leaves a lot to sort out. But, I propose two fundamental qualities. They are not unheard of, but, I think they will be effective in a number of useful ways.

One, use HP as a "mana" pool for con associated attacks. This offsets pumping con and any other defensive benefits associated with con such as regen, DR, or nat AC. This might work for defensive abilities also. The details can all be handled via options file so everyone can be happy in the comfort of their own computer.

Second, use a short timer because the 6 second round is a very boring eternity when all you have is one attack. I would go with a 1 second timer. Remember, the ability drains HP. Because of this, players will have to be careful that they don't kill themselves. I would not overlook self damage increments of 1 on such a fine time scale. NWN2 modules typically have a lot of combat. So, even at 1 HP damage per attack, it just might be possible to spam oneself to death, especially at low levels.

I think all other parameters can be tweaked to work with these two fundamental qualities to provide a new infusion of fun into NWN2 game play.

I would make a bunch of feats instead of a class and recycle current game elements: con based DC bonus, touch attacks, cone aoe, circle aoe, line aoe, chain aoe, force damage, sonic damage, stuns, knockdowns, bleeding, fear, haste, nat ac, DR, etc., so many options. Clearly, fighters and barbs are the first two candidates to use this. But, depending on details, other classes might get some useful mileage out of this.

Even the rational and background various feats can be re-purposed. First of all, con is a measure of stamina in D&D. So, using the powers requires stamina. There really does not have to be an RP rational of having a price for power. In, RL, stamina is well understood. That it would cost stamina to let out a negative energy burst shouldn't be hard to accept once you contemplate bursts of negative energy emanating from a person. "Mana" style power pools are fairly common in games anyway.