There's some pretty good series out there. None with such a large range and diversity as FF of course, but there's still a lot you may find worth seeking out:
Golden Dragon is probably the most similar series to FF and 2/3 of them were excellent. Pity there were only 6 in total. The combat system was simplistic, but actually pretty clever as it allowed the authors to (theoretically) make the combats fair while as FF combat always suffered from the large range of starting Skill scores.
Way of the Tiger is extremely well-written and the world building is excellent. They're very unfair at times though and just plain sloppy in others.
Lone Wolf also has excellent world-building and a fun continuous storyline. Combat was also much more fluid than FF though suffered even more from the problem that combats were either too easy or hard. The series got more formulaic as it went on unfortunately and some of the books were annoyingly linear.
Fabled Lands is excellent though very different from FF. It's basically the gamebook equivalent of Elder Scrolls. As I suppose is Destiny Quest, but that series focused far too much on number crunching for my tastes and wasn't as free as it first seemed.
I would recommend Tin Man Games' Gamebook Adventure app series though I'm not exactly unbiased there! They definitely owe a lot to FF, but the combat system is more involving and much better balanced.
I like the Endless Quest series but they're very much gameBOOKs as opposed to GAMEbooks. Often long bits of text between a decision and no dice, stats or note-taking.
The Adventures of Goldhawk are very Livingstonian (apart from the 4th one which to me reads like a Livingstone book but plays nothing like one). I like them, but I'm in a minority there.
I haven't played much of Blood Sword, but I've been fairly impressed with what I have played. The combat system is a pain in the rear though - I tend to use my own pared down version for convenience's sake.
Sagas of the Demonspawn seems to consist of well-written but long parts of text occasionally split up by long fights using the worst combat system I have ever come across: not only is it very cumbersome, it's also completely unbalanced.
Grailquest is funny, but the game system is pretty horrible.
I like the Sagard the Barbarian series but again I'm in a minority there. They're very combat heavy but I think the combat system is more involving than most series and they capture the Sword and Sorcery feeling well.
The
Alea Jacta Est series is excellent; well-designed, fun and more inventive than most.
The Sonic the Hedgehog series is a very mixed bag - the first two (by James Wallis) were excellent (if a tad linear). The next two by Nigel Gross and Jon Sutherland are the two worst pieces of utter tripe I've ever read. I'm not a fan of Sutherland's books in general, but these make his other books look like masterpieces. The next two (by FF's Marc Gascoigne and Jon Green) are fairly decent, but being mid-90s Jon Green they're incredibly unforgiving. Interestingly, Book 6 is extremely similar to Stormslayer in terms of plot and design.
Jamie Thomson's 2 Eternal Champions books are excellent. The combat is like a more involving version of Way of the Tiger's and they're pretty replayable. They also seem to have none of the sloppiness of some of his other works.
Virtual Reality is (mostly) excellent. The writing is more adult, and there's no need to worry about dice. Just don't bother with Book 3 if you value your sanity. Heart of Ice (Book 5) is a firm contender for greatest gamebook ever written.
The Marvel Superhero gamebooks are decent enough, some a lot better than others. They're maybe a bit too forgiving, but you can change that by pretty much ignoring the Karma rules.
Sherlock Holmes solo mysteries are interesting and very well written, but somehow missing a vital clue cos I rolled badly on the dice irks me more than getting killed by a monster for the same reason.
