I was planning on a split path like Indy, but there will prolly be a few random monsters that you have no choice but battle. At the moment, all 'boss' monsters will be beatable with logic as well, though.
As for the mythos of the KQ series, what's the point if I didn't?

I am currently writing the 'main path' down in narrative form, like the old King's Quest Companion, to keep my ideas on paper and just have a bit of fun with it. Once its finished, I will again try to make the backgrounds. Bt has offered some advice in their design, so it might be a bit easier. Here's an excerpt from that, in case you want to see it. Beware, its not really proofread yet, its only a first draft, and its a spoiler.
[spoiler]Connor was beginning to tire of this constant favor-doing. Could they not tell that he had to finish his quest? It was not important right now, he needed to get that mask piece anyway. He found a dry spot of land near the blackened water, retrieved his crossbow and tied a bean to the tip. Adding a drop of the fluid, he fired the arrow into the center of the sludge. There was a faint glow, then a green shoot blasted up from the murk. It curled and twisted, and suddenly it stopped, turning sooty black and noisily crackling like hot charcoal. He saw that his plan had worked, the piece of the Mask had stuck in a curl of the beanstalk.
He tapped the charred stalk with his staff to retrieve the piece, and the branch shattered like he imagined it would. The fragment fell into his hands and was soon nestled deep in his pack with the other. As he looked up, he saw an amazing sight. The blackness drew back like a great shadow over the swamp, pulling in its inky tendrils, soon closing in on itself. He had not thought that he would see so many astonishing things in his lifetime, if it had not been for this journey he would not have. In his astonishment, he very nearly lost his head.
Luckily he caught the squeak of metal behind him. Another henchman had materialized and was on the attack. Connor blocked the hacking blow with his staff, then swung the heavy wooden pole into the henchman's side. It seemed to destabilize the cohesion the pieces of armor had, but not enough. Connor got just far enough away to grab the firemushroom bag that he had. He poured the last drop of the growth elixir on it, then tossed it at the henchman. It slipped between the metal joints at its 'neck' and an instant later the armor exploded in all directions with a tremendous bang. He looked around, the armor seemed to be staying put. He grabbed the gauntlet that sat nearby and put it in his pack, figuring it might be of some use.[/spoiler]