The Unicron Trilogy
In 2002 Takara and Hasbro launched the first of three toy lines that were developed for release in both the US and Japan at roughly the same time. Each year featured its own gimmick. "Armada" focused on Mini-Cons, small Transformers that could attach to larger ones for extra abilities/weapons etc. "Energon" introduced a line of Autobots who could combine with each other while "Cybertron" brought on "Cyber Planet Keys" which you would insert into the figure to activate certain features. These three lines brought on new levels of innovation in the Transformers line that helped escalate interest leading up to the development of the live action movie.
Introducing a new play pattern, Mini-Cons attach to larger Transformers giving them additional firepower and unlocking abilities.
Combination is the key in this series featuring Autobots who can combine in dozens of combinations.
The Japanese releases of figures from the "Energon" toy line.
Cyber Planet Keys are introduced, each connecting to Transformers to unlock various powers including extra ballistic and melee weapons.
Galaxy Force (2004-2005)
The Japanese release of the "Transformers Cybertron" series.