Please do not brick back the dragons...
Ok. I know dragons will likely return at some point.
My suggestion is to not included them in the large castle sets ($100 USD) as they take away bricks from the castle. Including dragons in $30 or $50 sets is better.
Alternating Castle themes every three years from realistic (two human factions) and fantastic (one human faction vs undead, dwarves, dragons, etc) seems to work well. Fans can still find previous dragons or dragon sets on the secondary market for reasonable prices.