It seems that, ever since the Robots In Disguise/Car Robots line, the Autobots' leader has needed a 'Supermode' - an upgraded form, very much in the style of Power Rangers - to get by in the war against the Decepticons. Fire Convoy had an upgraded form of his own, and could combine with Ultra Magnus to form Omega Prime. Next, in Armada, the larger version of Optimus Prime came with a trailer which, depending on how one transformed the cab, would automatically become either a battle base or the so-called 'Super Pants' - a new set of legs for the super-sized Optimus. In Energon, Prime's trailer contained four smaller rescue vehicles - a submarine, a helicopter, a firetruck and a driller (much like the Mole in Thunderbirds) - which could attach to Prime's arms and legs in many different configurations to give him his Supermode. Now, in Galaxy Force, Prime has returned to the level of simplicity offered by the Robots In Disguise line - the basic robot transforms into a firetruck, and the 'Supermode' parts are made from the 'trailer'. Also, like RID, the trailer can turn into a small 'base' or gun emplacement, which can be operated by the basic robot.

This particular firetruck is far more complicated and detailed than the
rather basic RID one. He's all sleek angles and hard edges, with molded
detail everywhere

One popular features of the newer Convoy/Optimus Prime models - and
some of the reissues - is the Matrix Annex, which opens up to reveal
the Autobots' Matrix of Leadership, in one form or another. This one is
very similar to that which appeared in the movie, bar the colouring.
The best so far - and by far - would have to be the one that comes with
the 20th Anniversary model, which lights up (in the annex) and can even
open (damnit, open!)

But it doesn't end there. With the release of Ligerjack, it was shown
that the two could combine to give Prime a huge 'claw' arm. While the
cartoon showed this arm having real fingers, and while there were
rumours that the toy would have an attachment for this purpose, there
is no such attachment, and the 'claw' ends up looking very strange.
While most pictures I've seen show both of Ligerjack's heel pieces
folded out, it seems to me that having only one folded out gives him
the appearance of having two large, clawed 'fingers' and a thumb. The
Decepticon leaders Master Megatron/Galvatron also have an attachment
for Ligerjack's claw mode, and so Dark Ligerjack came into being...

But it doesn't end there either. Sonic Bomber, part of the Vanguard
Team, can also transform into a kind of 'Flight Mode' for Galaxy
Convoy. With Sonic Bomber attached in this way - with Convoy partway
between his basic robot mode and the Supermode - he becomes Sonic
Convoy, wielding all of Sonic Bomber's weapons, including his twin
swords.