Idea for red fossils in FFReforged, it's weird but hear me out.
The odd dna of a red fossil can apply to any vivosaur not just their respective species. Meaning in theory you can make hybrids of two, up to four vivosaurs. Think of red fossils as fun wild cards.
So while you could use the red fossils as regular and add some extra BP to your vivosaur, you could make em a hybrid.
Hybrids look suprisingly natural, just kinda odd. The head of course is the base species. By adding various red fossils, for example legs, then the vivosaurs legs will do one of two things.
Change to look closer to the donor. (Still remains functional, as other fossils seem to adapt around the red fossil to make the whole structure of the vivosaur more feasable...in otherwords your not gonna have a Base T-Rex with tiny little Guan legs. The legs will scale themselves up and the body and head a little down to come into proportion)
Or add abilities accociated with part. For example add a kicking vivosaurs legs to a traditionally ground stable vivosaur, suddenly the base vivosaur can kick well!
Sometimes both.
Hybrid vivosaurs do have downsides however. While they have more powers, the powers they get from the red donor are weaker, and as well weaken their own. The more red fossils the more "spread thin" your vivosaur is, so-to-say. So adding them is only recommend if you have the strategy to back it up.
TLDR: This is an excuse to make cool vivosaur fusions damn it!