To give Starcraft a bit of zing, here are a few rules you can try out for yourself:
T1 Units Only: Only "Tier 1 Units" are allowed in this game mode.
T2 Units Only: Only "Tier 2 Units" are allowed in this game mode.
T3 Units Only: Only "Tier 3 Units" are allowed in this game mode.
Harvesters Only: Only Harvesters are allowed in this game mode.
Air Units Only: Only Air Units are allowed in this game mode.
Fast Expansion Supreme: Must expand to a new base every five minutes in-game.
Ground Units Only: Only Ground Units are allowed in this game mode.
Tiers of units are are as follows:
Terran
Tier 1: Marine
Tier 2: Vulture
Tier 3: Bat
Zerg
Tier 1: Zergling
Tier 2: Hydralisk
Tier 3: Ultralisk
Protoss
Tier 1: Zealot
Tier 2: Scout
Tier 3: Carrier
(Note: If you get bored of PvP (Protoss vs Protoss), then you can always try something like, Cannons Only. Please note that this game mode could go on infinitly, therefore it is not recommended.)