It's great to see the interest in this concept. If there were a way to make it into a theme or build of another class I would be up for it, but it's such a unique idea to 4e there really is no parallel. An entire class might be beyond me, but we can at least come up with some interesting ideas to try out in our home games.
There were a couple comments, however, that the concept isn't clearly defined, so I wanted to take a step back and look at what makes an abjurer a, well,
abjurer, and then what makes it a
defender.
What makes an Abjurer an Abjurer?
Spells that protect, negate, block, and banish? That seems a reasonable place to start. I think there are certain classic D&D spells which are immediately recognizable as abjurations, so those need to be part of the abjurer's repertoire in 4e.
Also, my personal take is influenced by what is called "full contact magic" on TVtropes. I refuse to subject you to a link to TVtropes since it is a black hole for procrastination. Here's the summary: Gandalf vs. Saruman in Peter Jackson's movie, The Last Airbender, the mages of Dragon Age 2 - the use showy magic, often with a basis in force spells, which knock enemies around and feel extremely physical almost like a martial art. So that's where my placing an Abjurer as a build of the Swordmage came from. Kinda shallow, but there you go.
[sblock=Description of Abjurers from the SRD]Masters of protective magic, abjurers can fill many roles in a campaign, from bodyguard to battlemage to adventurer. Although it lacks offensive options, the school of abjuration provides some of the most effective combat spells in the game. An evoker or transmuter might find it easier to bring down large groups of foes, but an abjurer is far more likely to bring herself and her companions through a fight alive.
In a campaign with a high degree of intrigue or political machinations, the protective abilities of an abjurer can guard against foes both obvious and unexpected, and rival groups of abjurers, all with slightly different powers, might vie for prominence among each nation's schemers and courtiers. In combat-heavy campaigns, realms might employ abjuration specialists to protect against the battle magic of their rivals, and in any campaign, adventurers might come to rely on the skills of an abjurer to protect against the dangers of dungeon and wilderness. [/sblock]
[sblock=Classic D&D Abjuration Spells]
Banishment, Dismissal, and related spells
Dispel Magic
Forbiddance
Globe of Invulnerability
Glyph of Warding
Imprisonment
Magic Circle
Nondetection, Sequester, Mage's Private Sanctum, and similar spells
Prismatic Sphere/Wall
Protection from Evil
Remove Curse/Fear
Repulsion
Resistance
Shield
Spell Turning
Stoneskin[/sblock]
What makes an Abjurer a Defender?
This morning I re-read Schwalb's article on party building in Dragon #373 for the three main features of a defender, and thought about how the idea for an abjurer stacks up.
Black Hole: The core feature of a defender is that they lock down the enemies and keep them focused on the defender. Without that second part (something that was missing in my first take on an abjurer), you've basically got a quasi-controller. My thinking here was that the abjurer's wards on allies could be brought down by dealing sufficient damage to the abjurer or preventing the abjurer from sustaining them by hitting him with conditions like daze, stun, etc.
Enabler: A defender keeps the heat off allies so they can do what they do best, simple enough. The abjurer, being a ranged attacker, won't be using opportunity attacks in quite the same way, so finding a way to lock down small groups of enemies will take a different class feature.
Wall: Healing surges, HP, and AC (my thought was borrowing the swordmage's warding) are built in. However, the ranged abjurer needs a way to be constantly threatened like a fighter. Nemesis Destiny pointed out the warding should be tied to the abjurer's health in some manner. Also, there needs to be some mechanism for the abjurer to block movement (or at least make it dangerous) from range.