I think I saw a feat somewhere years ago that increased the distance I could use my Swordmage Aegis at, but I can't find it.
What is it called? Or was is a Paragon Path?
To my knowledge, there is nothing that turns Swordmage Aegis (any of them) into a Ranged power, so I'm going to answer this assuming you were just referring to increasing the distance between you and the target you are marking.
The two options for doing this are with Paragon Paths. Wandering Swordmage (FRPG p68) adds your Wisdom mod to the size of the burst at level 11 for Aegis of Shielding or Aegis of Assault, while Sharakim Blademaster (DrM386 p62) increases it to close burst 5 at level 16 for Aegis of Assault only.
As far as feats you might have been thinking of, Double Aegis is a Paragon feat that lets you mark 2 enemies in the burst with your Aegis, and Total Aegis is an Epic feat that lets you mark all enemies in the burst with your Aegis. And they do work with the expanding Paragon Paths, so an Epic Int/Wis Wandering Swordmage can potentially mark every enemy in a close burst 10. I have a friend who plays this build and it's pretty cool.
Edit: I was reminded in the comments of the Marjam's Dream Item Set. This set, if you have all 5 pieces, increases the burst size of any Aegis from 2 to 3. This is a very hefty investment for fairly small return, since the items in the set are not very good items.