You've picked up on something a few other readers seem to have missed...
SPOILER
It's not hard to complete The Dead World. The whole point of it is to gain a large boost to one of your stats. If you completed the book and didn't find anything like that, you indeed missed out on something. Since there are four directions you can explore, there are probably four types of power-up available...or maybe one direction is fruitless...I can't remember anymore.
In addition to the good advice above, I would add that you should perhaps think about the length of each adventure and begin with the one that is likely to be the shortest. My reading of the three ideas gives me the impression that: -The Dragon's Hoard would be the longest adventure with the most things to keep track of. -Halixor's Gauntlet can be as long as you want it to be, although the longer it is the 'richer' the story will need to be to avoid it just being a series of rooms that ends up becoming tedious. -Trial by Ice is the simplest premise and one that can work as a short adventure. I would avoid the temptation to do too much world-building and keep all of your story branches parallel rather than divergent.
Obviously, I am recommending Trial by Ice, although possibly lesser in scope than you may have been imagining (you can also create sequels to enlarge the world later, perhaps writing an entire series and maybe even finishing it instead of leaving everyone hanging...). To help limit the scope and add an interesting angle to the story, you could have the Champion of the Lightning Goddess look on the main character with utter contempt, offering no explanations, just a stipend to accompany him/her. Part of the plot can be uncovering what the Champion really wants you to do and why you were chosen. The point of all this or a similar idea is to give the story a central plot beyond the quest itself, and allows the plot development to be more than just a series of events/enemies/traps, etc.
I don't think the protagonist of Day in the Life is combating alcohol consumption, Inebriation is meant to be your high score. I had the idea for a Day in the Life II where our thirsty hero explores the outsider art scene of Wolverhampton and Birmingham, but meanwhile the second mission of New Day Rising is reaching within measurable distance of its end.
killed by a vampire, oh the indignity. this was a fun gamebook, well plotted. some of the text is a little hammy, but it doesn't hinder it once you get into it.