This is mostly a problem for 10M raiding since a lot of raids don't have a Holy Pally since there are 6 healing specs and only 2-3 slots. The fix is not to add another class, adding another class does not really help as 10M already have class juggling problems.
The most obvious option would be to have holy pallys get the armor bonus from wearing plate or mail. Then they could share gear with Ele/Res shaman, except for tier.
Another would be to have them wear DPS plate, have their spell power based on attack power (like ret) and have hit convert to spirit (reverse of druid, priest and shaman dps)
holy is better than it used to be for pally - we used to have a glyph of holy hand grenade to achieve aoe .. its kind of fine for some homogenization in the classes
it would be nice if they did make a use by adding more class specs - possibly making the different spec use different armor - to add a mail class over making a new class - a spec for a class to make use of the gear
subdividing would make it easier to balance over adding more - if adding a new spec for 11 classes you would still have more draw than an entirely new class - they did it with druid without too many people being upset ..
casting while moving would be a horrible idea - if melee weren't able to use ranged it would break pvp a little bit ... mind blasting from behind a pillar and popping out right when it finished - the reason casters cant cast on the move is because they are casting powerful spells - ranged almost always have higher damage output .. ive died because i hard cast a spell but it just meant .. i cant cast because i'm in the wrong place. .. if caster classes could do this aside from the fact they are OP .. i want a ranged move or a shield on a short cd to absorb it all .. spam my charge so i can freely move into the poor shadow priests face.. because priests and warlocks are so underscored in damage .. with all the different classes that can give a speed bonus while moving .. possibly the hunter one is the most confusing seeing as where it is tied to a chain of being disoriented while up if damage is taken - it should simply have a lower area of effect or recharge time - then druids. priests and so forth all have sprint bonuses - maybe even give it to a rogue for the sake of raiding ... it would definitely change the dynamic of how melee had to deal with them as ..ranged can very easily nuke a melee from 40 yards away .. the very least not give a bonus to all the classes to make other classes feel like things are being taken away from them
they used to have skirmishes which got taken back to wargames and unseen .. so ive said it before they should make that the spectator mode que for pvp and let people make friends doing that instead of trying to do it while getting points .. they used to be with the battle masters - that was nice too because it created artificial lobbies where you may even see the people before hand ..