And, as I suggested, devise ways to improve pets that DO take time and effort.
Pet Classes
Multi-zoned questing for items and/or spells that improve individual casters. It would force people to focus on ONE character at a time, rather than just loading up 30 chars and going afk in t3/t4 during double loot. If you want the items to be scaling through tier, Halloween type spawns would be great. Collect an item from each spawn in each zone, combine to make an awesome item. The LDoN Mage robe is a good example of an item that provides a nice little burst of DPS, but requires you to pay attention to the timer and have it unbagged to use it. Maybe a 2 minute, pet only, holy knightstrike proc with 5 minute cool down? Extended LDoN buff? Things like that would certainly make me spend a little more time and pay a little more attention to my Mages.
The idea of making something harder to play, but better overall breaks up the traditional "You should be grouping X, Y, Y,Z,Z,Z" formula and gives people a real decision to make when forming their groups. This has been done pretty well recently with Wizards/Bards. Harder to obtain is one thing, but there's always going to be a handful of people who will do it relatively quickly and then put the character on auto-pilot from there on out. Harder to play is the limiting factor.
This is coming from someone who has 3 mages and a Zerker simply because I haven't felt like doing Augs yet. I know after a certain point this is not viable DPS and I'm already working on Monks, etc., but being able to have a viable mage or two after doing some aug-like questing would be pretty cool.