A "shield" around the swordmage also works.I vaguely remember we houseruled the swordmage teleport attack to be an easy to apply and track aura instead of individual enemy mark tracking after essentials came out and did that for other defenders.
I am a big fan of the essentials aura defender mechanic compared to the individual enemy marking.
In 4e, with the myriad of defender marks, the swordmage's Aegis was interesting in that it let you, nay encouraged you, to mark a foe and move away from them. You'd then use your presence somewhere else to tank, plus interfere with one foe's ability to fight your allies.
The swordmage had to use tools to (a) get close to the foe, (b) get away from the foe, (c) engage another foe. Which was the swordmage's minigame. (Good 4e classes had tactical minigames; ranger baiting, rogue getting advantage, avenger isolating a foe, defenders and their marks, warlock boon triggering, controllers and zones/positioning of enemies, leaders and producing bulwarks and positioning allies; this was layered on top of the default 4e tactical combat game.)
Your aura solution also reduces this effect; either it is short ranged (making the swordmage mark just another marking mechanic, nothing different) or long range (making it nearly unavoidable).
But in 5e, there isn't a myriad of mark mechanics. So the aegis doesn't have to fit in with them nor stand out from them.
The reason why I proposed (the honestly weaker) protect an ally instead of mark a foe is that it fits the pattern of 5e protection abilities better, and being less tactically dominant it also fits the generally weaker 5e defence mechanics. In 4e, monsters tended to have abilities explicitly designed to counter defender marks (foes marked defenders, explicitly shed or avoided marks, etc); in 5e these are less common (mostly just flyby, honestly).
Hence the weaker aegis.