I don't know, Earth and Air can fit in with the fire theme pretty well (magma and smoke leap immediately to mind), or you could even go with your own Fire-focused elemental system.
True, but to be honest those things are already implied as part of the fire element, even though there is some cross-over with the other elements. For instance lightning is a cross over of both earth and air and so the book specifically states you can create lightning using either element on it's own. In the same way ice at first glance seems to be water magic, however ice can be created using fire magic to draw the heat out of something.
Admittedly, a user of both fire and earth could manipulate magma more, creating molten rock out of thin air rather than heating already existing rock, but I would personally allow the scion of the fire god to do that anyway in the same way as I'd let a water mage create ice out of thin air without knowledge of the fire element. It'd replace his ability to draw heat from objects, assuming a fire-based scion can only add more heat to an object.
As luminos says, it's just a guideline.
Oh and rickayelm, also note that a breathweapon power can be used to create manoeuvres as well as attack, so if you want your character to be able to create fire without taking mental stress that could be an option.
