Please, make more Volumes for the Monster Manual, I will buy every single one of them!
I'm not the kind of that buys in to the fluff. I do not care for pets, as I see it, their only purpose is to clutter up my screen. Neither do I like buying stuff that boosts your characters, like tomes of Fate and those new skill tomes, as they defeat the purpose of actually playing the game to me. Power should be gained through gameplay, not credit card.
What I do want to buy is of course, content (quests, races, classes), but these take a lot more time to make, that I understand. What doesn't seem to take very long to make is Volumes for the Monster Manual. It seems like one volume per update wouldn't be that much effort on your part now that the mechanic is in place. A lot of players are interested in them, either for the lore, the stats information, the small xp bonuses, the fun of finding every single type of a monster, or simply because we like to have a tracker for our kill count.
These are low-hanging fruits for you guys, as much as pets, make the most of it. A lot of us are ready to pay for these and sadly you aren't releasing any. Please take our money.