We'll need more information, and ideally a decklist to "edit" before we can help more. Information like type of multiplayer games (chaos, teams, etc), and your card pool. Since you are aiming for fun, your metagame is unnecessary for now.
Anyways, go five color, heavy on the red, splashing the other color. Use single copies of the Invasion dragons, but avoid the five Elder Dragon Legends (well, except maybe Nicol Bolas), as the upkeep on them is atrocious. To keep the deck a decent size, just run one of each other dragon printed (use multiples for a thicker deck). Make sure the legend that fetches dragons is in the deck, as well as enough creature search cards (Worldly Tutor and perhaps Pack Hunt, among others - mostly green, though) to fetch more dragons.
For more flavor, consider including cards that either mention one of the dragons in title or flavor text (Darigaaz' Breath), or feature dragons in the artwork (Fiery Covenant). This will let you fill in holes in the functionality of the deck, while still staying on theme.
As for land, well, the assorted Lairs are automatic, as well as enough multicolor lands (duals are best...) to consistently play everything.
Oh, and beware Rashida Scalebane!
