Well, me first (duh):
1. Barrier: I've always wanted to see a "impenetrable barrier" GP. Available in heroic or mythic or something, you select the GP and then paint over the landscape you want to cast it on. The maximum total length would be about 8 large wall sections. Then the forcefield would last about 30 or 40 seconds before vanishing, and NOTHING can pass through the barrier while it's up. Good for building important buildings, or buying some free Wonder protection time.
Although there'd have to be a way to, well, AoM-ify it... A forcefield fits more into StarCraft than AoM :P
2. No Name: A Greek GP available through Hades. It works much like Flaming Weapons. However, instead of doubling attack power, it doubles the number of projectiles fired, be it ship, building, siege weapon, myth unit, or human unit, and doubles the range of human and myth units. This affects your entire team and lasts 60 seconds.
3. Guidance: Based off of #2, exept it simply makes it so that no attack can miss. Arrows and other projectiles actually seek out the target. This is deadly with Manticores, because usually only about 1/2 of their projectiles hit their mark.
4. Wanderer: Simple Archaic GP. Makes the target unit invincible for 20 seconds. Works on all units exept Titans. Very versatile.
"Build a man a fire, and you warm him for a day. Set a man on fire, and you warm him for the rest of his life." -Chinese proverb
"If MacGyver and Jack Bauer were in a fight, Chuck Norris would win."