Well for now pets are just decoratives and RP sadly.
Maybe new age things are gonna be different.
As for today regular version of pets just pointless, greater are better, but not worth investing time to 20 lvl ups (thralls are much better). Pvp\Pve no matter - all pets just for RP thats all.
T3-4 NPCs and bosses hitting them like a trucks.
Better pet system would be smth like this:
-all pets starts with regular version (no greather variants at all), that how you give them value
-regular pets get buffs at stats + some new abilities (passive and active)
-to convert pet into elite version you must reach certain lvl and criteria (rare resources needed)
-to convert pet into epic\mini boss version you must reach certain lvl and criteria (very expensive)
So basically pet follower gonna have not 2 but 3 tiers (like scorpion pet for ex).
Regular, elite, boss. Better tier have better stats + better expanded abilities and buffs.
Each tier with own model and appearance + some unique animations, sounds and effects.
To much work for single update i know, so maybe Age of Beasts then?