SaGa Frontier can be considered a fusion of the two earlier branches of the SaGa family tree, which had very little in common with each other apart from the name SaGa and being developed by the same studio (namely Square's 2nd Division).
The SaGa series, also known as "Final Fantasy Legend", appeared on the Gameboy portable console. The two SaGa games (yes, there was a SaGa 3, but it was not developed by the 2nd Division and thus was not a "real" SaGa game, just as Final Fantasy: Mystic Quest is not considered a "real" FF game) had two distinguishing characteristics.
First, they took place in a "world of worlds": instead of a single world map, there were many small worlds connected to each other, each world with its own flavour (one world in SaGa 1 was mostly ocean, with its inhabitants living on scattered islands; another world in the same game was a post-apocalyptic urban wasteland)
Second, the
player character "classes" available were... unorthodox. While most console RPGs of that era had characters based on some variation of the
D&D archetypes of
Fighter,
Mage,
Cleric and
Thief, SaGa's four classes were actually four entirely different types of lifeform:
Humans,
Espers,
Monsters and
Mecha (SaGa 1 had only the former three; Mecha didn't appear until SaGa 2)
The Romancing SaGa series, which appeared on the Super Famicom and only in Japan, was totally different. The Romancing SaGa games took place in quasi-medieval sword-and-sorcery worlds and had only humans as player characters. The Romancing SaGa games had two highly distinguishing features. First was the Free Scenario System. Unlike most console RPGs in which a scripted story constrains the player to visit the game's locations in a preset order, in the Romancing SaGa games it was possible to explore the game world more or less freely. This system gave the games a greater resemblance to PC RPGs such as Ultima than to other console RPGs such as Final Fantasy. Romancing SaGa 1 and 3 even allowed the player to choose from eight different main characters, each of which began the game in an entirely different location, with different scenarios and allies available.
The second distinguishing feature of the Romancing SaGa games was the weapon skill system. Unlike the vast majority of console RPGs (and pen-and-paper RPGs for that matter) in which different weapons are distinguished only by having more or less "attack power", the Romancing SaGa games featured eight different weapon types (sword, two-handed sword, axe, mace, spear, rapier, bow, bare hands) each of which had a variety of special techniques. These special techniques could be learned spontaneously and randomly in the middle of a battle: a "eureka" lightbulb would flash over a character's head just prior to their turn, and the character would announce and execute their newly-learned technique. These weapon techniques tended to look quite flashy and gave the Romancing SaGa games the same whizz-bang appeal that summoning magic gave to the Final Fantasy series. There was also a defensive counterpart to this lightbulb system. Occasionally a character would "see through" ("mikiru" in Japanese) an enemy's attack; a lightbulb would flash, the character would dodge the attack, and from then on that character would be totally immune to that particular attack type.
Anyway, SaGa Frontier attempted to fuse the best parts of both worlds. Like the Gameboy games, it had a "world of worlds" setting and four different player character types (Humans, Mystics, Monsters and Mecha) Overall, SaGa Frontier retained the goofy "anything goes" atmosphere of the GB games over the "serious" medieval fantasy of Romancing SaGa.
What SaGa Frontier took from Romancing SaGa was the Free Scenario System (including seven different main characters to choose from) and a complex skill system which combined Romancing SaGa's lightbulbs with some entirely new mechanics. The lightbulb system was still in place, but was exclusive to humans. The other character classes learned their skills by different means. Mystics "possessed" enemy monsters, making one of that monster's skills their own. Monsters actually ate enemy monsters, absorbing the victim's powers and sometimes causing the monster to transform into an entirely new form. Mecha downloaded "programs" which increased their offensive or defensive abilities or gave them entirely new powers.