Some thoughts on pets...
The way I see it, there's basically five kinds of pet, and they're all different. Keep in mind this is all my own opinion.
Tank PetsThese pets keep the mobs from hitting you while you nuke them down. IMO, they should be tough enough, and be given enough hate/lifetap procs, that they can survive and hold aggro admirably against one or two tier-appropriate raid trash when properly focused, geared and buffed, but have little actual DPS, relying on their master for that. Using a tank pet forces you to sacrifice DPS for self-sufficiency & versatility - a tank pet can offtank adds so long as no melee DPS gets involved (since mobs target players preferentially to pets).
Examples:- Mage Earth & Water pets
- Necromancer Skeleton pets
- Cleric Animated Weapons
DPS PetsEssentially, half a rogue/wizard/berserker. These pets don't tank and should die if they try, but should be capable of surviving stray hits/ramps. They kick out damage comparable with their master's own. Pet + master should come close to the DPS output of a dedicated straight-DPS class.
Examples:- Necromancer Spectre/Shade pets, plus Vampire & Zombie T8
- Mage Air & Fire pets
- Shaman spirit wolves
Swarm PetsBasically a DoT that can be killed by AoEs and forces enemies to waste effort splatting them. IMO most swarm pets do not have a long enough duration. These guys are fire-and-forget and totally expendable. In a pinch, they can intercept a mob that's out of control, buying a few precious seconds for an actual tank or tank-pet to take up the mob.
Examples:Charm PetsExpendable! Unlike a real pet, you don't need to support or take care of 'em. You charm 'em, you fight alongside 'em until they die or break free, and then you charm a replacement (which, if it broke free, might be the original mob). Other than their expendable nature, they're otherwise similar to a tank pet: perhaps charmers can be given a buff they can cast ONLY on charmed mobs, granting that mob some melee mitigation, hate generation, and a lifetap proc on melee attacks? Maybe even incorporate it into the charm itself? That way, no matter what mob you take over, he's guaranteed to at least hold aggro until he dies, and will die a bit slower than its uncharmed counterpart.
Examples:- Anything an enchanter or bard charms.
SidekicksAssistants to another class. These pets are meant to fight alongside another class in melee, and so are generally like a DPS pet in general behavior (since mobs target players in preference to pets, these pets cannot tank so long as their master is in melee with the same enemy). The pets should be adjusted to support their master in some way, and vice versa; the two will be significantly weaker without one another's help. Perhaps a pet that procs/casts buffs on its master? Maybe a pet that sings bard-like songs for master and allies?
Examples:- Beastlord animal warders, Shadowknight skeletons