For example, with High Alert in play, Shield Sphere now functions as a 6/6 for zero mana
Thats not an essential part of any crossover, but you cant argue that Godzilla, The Walking Dead and Doctor Who fit together in any logical way
If you want an X-spell that converts all your spare mana into tokens, you cant do better than Forth Eorlingas!, which not only spits out hasty, trampling tokens but gives you the monarchy
It is very splashable both in white and black decks, the latter due to heavy numbers of good discard outlets enabling you to play it even without any white mana
Cards that are pricier and provide less impact than others are good places to start: Foray of Orcs, and Fire of Orthanc stand out as over costed removal options in the deck