FIST Avalon is an interesting exercise in bending FIST systems into new shapes and possibilities.
More than altering, Avalon extends many of the core concepts of FIST. Roles become two sided with a vice and virtue, that not only rewards role playing but challenges players to follow their knightly oaths to avoid negative effects. To go along with this change, the included mission creation rules recommend adding these virtues as constant themes.
Traits with their own advancement trees have been added, including permanent and temporary bonuses for adding new traits and swapping old ones out. This adds another layer to character growth that encourages a character trying new traits to gain power.
Avalon adds a different crunchiness to FIST that those who are seeking out more character building options will find enjoyable.
On the other hand, its ethos might run counter to those who enjoy FIST for its relative character minimalism and high lethality. By increasing both character build options and complexity, it becomes a harder pill to swallow when your character inevitably dies or if you are consistently having to make new ones.
If you are looking to add more mechanical complexity to character building or bring knighthood into the forefront of play, Avalon will give you a lot of material to run with.