AI script will first do a step forward, especially for monsters with special abilities or spells.
Some monsters will be weaker, because they were more powerful than in P&P. However, they might gain P&P abilities that help a bit.

Globally, this is an improvement for most monsters. They will be stronger, smarter, and with most of their P&P abilities.
Smarter script help a lot monsters with many abilities (succubus, sirene, hamadryad, demons, ogre-mage). For all others, it's more a matter of beeing able to track, rest and recover, or pick easiest target.
All in all, I'm pretty satisfied with them. Now, I will use a lot more shapeshifting abilities or conjure monsters & animals.

It will be no more possible to kill a polar bear with a solo level 2-3 character, using classical hit & run tactics.
However, a lone monster will not increase difficulty by much (except if you take a very powerful one). But you will see a difference in big fights with many monsters.
That beeing said, players like me who used to play solo will certainly see a difference. It was possible to take most of monsters solo with cheesy tactics and gain lots of experience.
But it doesn't make sense to see a lone level 2-3 warrior killing greater basilisk, ankheg, doom guards, sirenes. In P&P, it wouldn't have been possible!

Ankheg: one acid spit per day, entangle target with no save on hit.

Basilisk: petrification is area of effect (as in P&P)

Greater Basilisk: they were just basilisk with more HP! Now, they have their foul breath on melee range (save or die) and poison claws. They are also strong in melee with heavy HP.

Bears: they are all a lot more stronger and faster! This was their big weakness... Bears move as fast as human in P&P, why not in game?

Carrion crawler : they now have their 8 attack per round. Their ability to hold with smarter AI and high number of attack make them more powerful. but they are still easy to kill (very low HP)

To be continued...

Relating to P&P, Monsters will be based on 2nd and/or 3rd edition. It depends on what I can find or what I can do.
Overall, 3rd are better and I will try to stick to it when possible.

I am making a new tool which will allow me to add special abilities in a very easy way. I will have a very good control on every monsters.
From actual standpoint, it will be a major change. I expect to produce a high standing component.

I fully understand this is your mod and you will make the way you want. BG1&2 are 2nd Ed. AD&D games and I hope it will stay that way. That is why I play it not 3rd Ed. based CRPGs. Not that everything 3rd Ed. is bad, but 2nd Ed. is a fully developed game. If you are going to use P&P rules to backup your ideas then sticking to one game would be best.

I like 2nd rules and to be honest, I didn't play NwN because I didn't want to learn 3rd rules :-)

My concern is for monsters. 2nd monsters book is sometimes poor regarding monsters. They only give intelligence score to start with.
I prefer 3rd monsters book for their really detailed description and abilities.

I will make monsters closer to 3rd than 2nd. Most of the time, difference will be very small. But... I really like how it is done. For example, a Helmed horror is flagged as Construct and we find a list of all immunities related to construct.
They usually have more special abilities and they are more detailed. For me, it is important, because I like to see a monster with its own identity and abilities. Not just a different model and slightly different numbers.

For every other things than monsters, I will stick to 2nd !
If you don't like 3rd edition monsters, just don't install that component Maybe I will make 2nd edition monsters as well. So players can choose what they want.

Chev I agree with you but you must keep in mind that having monsters being inspired b the 3rd edition rules is just a sort of conversion of the P&P features of the 3rd. Edition Monster.

I believe it's a very good idea because, as aigleborgne said, it's then possible to give a more precise identity to monsters, thanks to the more material avaiable for its P&P creation.

At the end, I would guess that there will be some sort of mix between 2nd and 3rd edition P&P source.

In sum, this is probably best. The example for helmed horrors is apt, because they weren't AFAIK in 2e, so we must rely on 3e stats (and I think I coded this up somewhere or at least listed what the stats should be in the G3 BG1 forum). A bunch of 3e stuff anyhow is not codable in IE, being 2e based as Chev says. So where there are 2e stats (not always, as with helmed horrors) those are usually more reliable. I look forward to seeing what you come up with anyway. A lot of stuff I did originally for another mod would probably fit better in a mod of this nature.

Check this: http://en.wikipedia.org/wiki/Construct_(Du..._&_Dragons)
Helmed Horrors do exist in 2nd edition.

As for a mix, I will always start from 2nd edition and use 3rd editions to complete data or change some that I couldn't do from 2nd.