Unlimited power with SRD only materials

[gooddragon1]

I think the green text needs to be read in the context of the paragraph. It goes on and specifies what is meant by adding new magical abilities - defined magical abilities the magic item didn't have before, in the example making a +1 vorpal sword into a +2 vorpal sword. This doesn't refer to adding completely new effects at all.

The only way to ad hoc your wish would be to create a custom item with a newly created spell that does what you want to achieve, and the DM can disallow new spells he finds to powerful (DMG p. 35 et seq., ELH p. 88 et seq.). And this is assuming a very lenient DM, a less lenient DM would probably rule that to achieve your wish, it would have to create a new spell as part of its effect, which is not within the power of the Wish spell, so it falls under the greater effects clause.

You are ignoring all spell requirements as part of the ignoring of prerequisites to create a magic item except XP.

Helm of Brilliance

It emanates a bluish light when undead are within 30 feet. This light causes 1d6 points of damage per round to all such creatures within that range.

What spell in this list...:

Strong varied; CL 13th; Craft Wondrous Item, detect undead, fireball, flame blade, light, prismatic spray, protection from energy, wall of fire; Price 125,000 gp; Weight 3 lb.

Does exactly that?

None of them.

By extension, no, the effect does not need to be existing. You can create entirely new ones without a new spell.

In fact:

Silversheen

This substance can be applied to a weapon as a standard action. It will give the weapon the properties of alchemical silver for 1 hour, replacing the properties of any other special material it might have. One vial will coat a single melee weapon or 20 units of ammunition.

Faint transmutation; CL 5th; Craft Wondrous Item; Price 250 gp.

Note the lack of a spell requirement.