Just as an exercise, I thought I'd think about how much work would need to be done for this.
At the end, I think surprisingly little. Most of it would be in balancing the damage.
First, we'd have to recognise that the spec has to have a shield and 1H weapon. That improves armour and adds stats. Would their be DPS shields? Or would the class make use of the defensive stats and procs of tanking /healing shields?
Second...the toolkit.
Prots toolkit is suprisingly complete. However, it would need to borrow from Rets to provide a truly meaningful DPS toolkit. BTW - I'm basing this on the Cataclsym Paladin.
Single Target
Crusader Strike
Shield of the Righteous
AoE
Hammer of the Righteous
Divine Storm
Consceration
Ranged
Exorcism
Judgement
Avengers Shields
I would like to toss HShk in here as well as a ranged HP generator.
Utility
Holy Wrath
Hammer of Justice
Rebuke - requires Shields?
Procs
Divine Purpose
Art of War
Cooldowns
Avenging Wrath
Guardian of the Ancient Kings
You'd also want to have a look to see which moves can be converted to require shields to integarte that aspect into the look. Or add new ones - e.g. Spellcatcher: charge to shield to store the next direct damge spell and then fire it back at your current target.
Third - the effect on tanking
Yes, I feel you would need to take into account tanking.
There are several ways that this could be done.
1: Tanking moves require a 2H weapon.
2: A stance mechanic
3: Off role tanking is possible - e.g. ShoR gives 15% block mechanic in addition to damage while TV returns as the single target DPS finsiher - but raid tanking requires appropriate gear and talents.
4: There are a couple of other ways to address this issue
Or a mix....separate the roles (via stance and weapon mechanic) but you may want Rets capable of tanking dungeons (include SHoR and a talent allowing 2H wielding with shield)
EJL