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As I see the problem with the map right now is that mages are too **** slow to do anything else but defend a point. Unfortunately, this stacks with another problem mages have on the map.
If you're an ultimate reliant mage, your cooldowns are going to be too long to contribute much to the game and there's no golem buff to help alieviate the problem. There's also a much lower magic penetration buff than armor pen buff.
The result is that mages have to use a cdr + magic pen build that overall gimps their damage output.
In addition, the slower mages like Veigar or even Annie and Brand end up like the fat kid at the back of the line yelling "Hey guise, wait for me!" while even the slowest DPS stack zeals or phantom dancers and the slowest tanks put on their Force-A-Natures and start zooming around the map.
Mage assassins (Kassadin, Akali) or area denial mages (Heimerdinger) do very well on the map. Pure mages with a few exceptions do not do so well.
So my proposed suggestion is this: An Mid-Tier (Advanced) AP item that builds out of Amplifying tome that gives small (25ish) Ability Power, 5% cooldown reduction, and 5% movement speed.
This would combine with Haunting Guise into a Legendary item that gives something along the lines of
+30 Magic Penetration
+50 Ability Power
+10% Cooldown Reduction
+10% Movement Speed
As always, most of the effects of the new items would be unique, the two new iems would be limited to Dominion mode, and the whole thing would be priced to compete with Ghostblade, Force Of Nature, and Phantom Dancer -etc. etc. And if this causes this + Lich Bane to be too powerful of a combination we can always remove Lich Bane from Dominion or include a "Mini-Lichbane" in it's place akin to Kitae's Bloodrazor.
TLDR: Make Haunting Guise build into a magic pen + mobility + cdr item for the slow-ass mages on Dominion.