Above greatwyrm you're in dragon god territory. Dragons that powerful should, IMO, be unique creatures and you can give them stats similar to a deity, archfiend, or other power entity.
As an example, the greatwyrm red is a mythic monster. You can make it more powerful just by giving it more mythic stages, increase its CR, or both. I guess following the pattern I've started:
Greatwrym (mythic)
Minor Dragon god*: higher CR, probably about 27-31 depending on type, (i.e. a CR 31 red should be more powerful than a CR 25 mythic red)
Lesser Dragon god*: higher CR 28-32 + mythic
Intermediate Dragon god*: Higher CR 35-39
Major Dragon god*: Higher CR 36-40 + mythic
Greater Dragon god*: Higher CR 43-47
Great Dragon: Highest CR 47-50 + Mythic
Of course you could use multiple stages of mythic instead.
*dragon gods are not true deities, they are just really old and powerful dragons that rule over their lesser kin, IMO.