I think it would be better if up to 50% of the action points carried over to next turn, otherwise, it makes some weapons really weaker (the bill for instance, forces the used to waste 33% of his action points doing nothing).
I thought about this as well. Wouldn't it be better to add them as a bonus to defense or something similar, though? So you couldn't wait-stack AP making for even more HoMMish space-clicking..
Don't think of the two-handed weapons wasting action points after attacking - we could just increase the AP cost for using the skills and there'd be no waste. Instead, consider this: We wanted one-handed and two-handed weapons to feel different in their use, which is why we decided on two possible attacks with one-handed weapons per turn, and just one with two-handed weapons. One-handed weapons offer more flexibility, especially combined with a shield (two offensive actions? one offensive + one defensive? two defensive?), whereas two-handed weapons are more high-risk high-reward in terms of less defense and more damage (or in the case of the Billhook, utility) with just one shot per turn. We settled on an AP cost of just 6 (instead of 8 or so) to give the two-handed weapon wielder the ability to move one tile in addition to his attack, just as a bonus.
Speaking in general, however, we considered both your options, carrying over action points into the next turn and having a bonus to defense. The latter option we liked more but ultimately also kind-of decided against. As you know, our characters' actions are based on the use of skills - and ending a turn then to receive an effect not from a skill, but from a rule existing in parallel to our skill system isn't the most intuitively fitting thing. Instead, having a bunch of defensive-minded skills (Shieldwall, Spearwall and Riposte currently) proved to be a much more clear and interesting approach to enable the player to invest AP for a defensive boost while at the same time also giving more fine control. As we go on to add more weapons that didn't make it into the demo (throwing weapons, two-handed axe, warhammer, etc) and refine some of the already existing ones we might also add more unique defensive skills. Possibly even to one or two of the two-handed weapons to better distinguish them from another.