Currently the semi mega beasts are made up of various naked unarmored giants who are rather underwhelming. What if these giants could be "culturally" diverse?
1. An easy one is to allow them to be armed and armored.
1.1. Savage ones might use a maul (or tree) and be clad in hide armor (would probably be leather if using the current set). The cyclops should probably throw boulders (but current mechanics might make that problematic).
1.2. More advanced giants would use more advanced weapons and armor, especially if backed by an entity/civ or sire.
2. (Some) giants could literally be semi divine and either act as they like, or be some kind of champion of their deity (possibly depending on if their creation was intentional or just the side effect of a fire-and-forget affair with a mortal).
3. Giants could be allowed to take over communities and/or allow/foster cults around themselves. This could naturally lead to a "giant" invasion where the invader is actually just the leader of a band of invaders. They might actually form new civilization entities from conquered or abandoned (by other civs, but not the inhabitants) sites allowing for the generation of new civs, not just the destruction of a world generation time produced set.
4. Giants could be allowed to be civilized, and visit your fortress as scholars (looking for the location of a mate in your archives?), or possibly even join the fortress (would probably require that the issue with "wild" animal people etc. to be solved first, though).
5. Magic wielding giants could be truly terrifying...
6. If the semi mega beast set was modified in this kind of way, not all of them need to be giants, but could be formidable intelligent humanoid creatures of other kinds (semi divine, again, might allow for some rather formidable creatures that aren't huge).
7. Many/most giants might be dim witted, but some fully intelligent ones would be nice, and might make some of the above easier.