Winning a Master Quest or Spell of Making victory is pretty neat. Each describes the immediate creation of two scenarios which would seem fun to play out... but then the credits just roll.

Wouldn't it be nice if we could keep playing after the credits, in the resulting world that we've created?

Master Quest:

Lots of new dragons spawn on the map, and they, as well as all existing ones, become a new dragon faction, which is permanantly allied to you. They will roam the world deliberately attacking your enemies and crushing their armies, thus allowing you to (if you choose) sit back and watch as your new army tears across the land, razing every enemy city they take.

In addition, you would also gain a new Rod of Dominion spell. With no manacost or cooldown. Casting it on a dragon would allow you to immediately take "direct" control of it, meaning that it would be "your" unit rather than an alllied one, letting you put it in armies, command it directly, etc. The purpose of this mechanic would be so that you don't end up suddenly having to give orders to 100 new units, which would get boring fast.

From there you could just go on to conquer the world and lay waste to everything ^^

Spell of Making:

You immediately and permanantly take control of all mana shards in the world (even in other people's territory). The appropriate "Temple" appears on each of them, and becomes indestructible, so your mana shards cannot be pillaged. all other players are reduced to 0 mana and can no longer own any mana shards, or cast any spells.  Your sovereign and all your champions immediately gain Archmage promotion in every school of magic. You also get a permanant +400% boost to research. 

Proceed to doing whatever you see fit with your new and nearly limitless mana reserves and massive power from all those shards. Cast curgen's volcano 50 times if you like. It'd suddenly turn the game into a magical sandbox, which would be interesting.