So here is a basic start. I've used Andy's body and added five different types of head on them. Using the D&D good ol' standard of classification, I am going to make them Hill, Fire, Frost, Stone, and Storm giants, from left to right. I will make a few of each, and work on a uniting color scheme for each group
Here you can see my whole troupe. I did a different color wash on each group to give them a little unique character (beyond the hair color). The hillies got a brown wash; fire giants, red; frosters, dark blue; stonies grey, and the storm giant, white (which creates an inverse shadow, kind of like a glowing or effervescent figure). Everyone but the stonies got some clothes other than Andy's Speedoes. Since these are all meant to be simple garments, for the hill, fire, and frost, I used carpet sample swatches. For the storm giant, I added a papier-mâché toga. Industrial tile glue holds them in place - acrylic seal keeps it all together and solid.
Now for a little more exotic go, I am also adding another bit, the HorrorClix Devil Imp. Again, cheap and plentiful. And that big head will look nice up on some 3" tall figures' shoulders.
Here you can see some in progress. I added a couple of GW demon-something arms. One figure gets a left, another a right, and the boss, both. I've also added a couple MechWarrior bodies under the giant heads, complete with leftover car door shields. (Let's see how that works...) Down in front, I have added the GW ogre heads to some of the bulkier HeroClix ... Incredible Hulks, the Blob, etc. And I put a couple snake heads on the Devil Imp bodies. No sense wasting those when I can get a bag of a dozen snakes for a dollar (and the bodies will make nice tentacles for something else, later on).
Here are the Efreet giants. I got the translucent fire head off of another giant Heroclix figure. (You might have noticed it in the last pic ... also it's hands.) All in all, I think the head goes well on this figure and the spandex armour bronzes up fairly decently.
And the more traditional giants. This guy got the burning hands. I attach transulcent pieces after sealing by scraping the joint down to bare plastic and using industrial adhesive. In this case, the joint is so large (almost 1/4" diameter), that it is very easy to do.
The mechabodies don't look too bad, either. There are a couple of places where the joints are too thin to realistically be a giant dude in some type of armour. But overall, it's a nice effect. If I want, I can go back an putty in some bulk and then cover it with some additional armour plates. That is, little pieces of cereal box.