Im really glad you asked because I have a ton of ideas.
Firstly you have to decide what audience you are trying to appease. Will you make it more story based and concise or allow it to kind of branch out and wait until the end to reach the climax. I personally just beat the quest and I would like to say that while the story was good, I really like it when I can somewhat have a better idea or slight idea where the real story is. You also have to decide how much like the quest you will make it. Are you going to make it where you can solve quests in different ways. Those are really cool ideas and some of my favorites as it warrants for your character to have more replay ability with future characters.
Something else to consider would be how difficult are you going to make it. One of the biggest problems with certain games is that certain ways of combat are way better than others (melee vs ranged). Its not the biggest deal but when you can't get past a part in a game because it was made nearly impossible for your build, it will tick you off, trust me.
For me, I just beat the quest base game and think that I did a fairly ok job at making choices for my character but I rushed it to the end of the game and that was just because I wanted to see the story unfold. If you have story like that, its not bad, but when melee is not the most tactical side of combat, it can get a little boring.
I would say to go for a medium 25-30 hour or less scenario and hear me out on this. I recently completed Avernum Escape from the Pit with 50 hours into it. While I was playing the game all I could of later was going to Avadon which was much shorter. Reason? Avadon was more concise and was tough but not ridiculous, it threw a good practical story at you without you having a choice but to read it, and because generally everything connected more.
You don't have to listen to me on any of this, but those are just the things that I think make games great.

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