If you save every gold you have right now until the next expansion, you won't actually be any poorer than you currently are, but you sure will feel like you are. Assuming there are no new gold sinks, the amount of gold entering your characters' pockets is going to skyrocket, and they will all buy stuff you want, raising prices across the board. Farmers will make more from crafters, who will charge more for crafted goods.
World of Inflationcraft
Inflation is one of those universal truths like taxes and hunter envy. Unlike hunter envy, though, it can't be avoided by simply upgrading your main from a support class to a hunter. The best way to deal with it is to plan ahead and tie up as much of your money on things that will go up in price. You want to aim for things that will rise at least as much as inflation will erode the value of your savings.
If instead of worrying about the value of the gold under your mattress, you're living paycheck to paycheck, you're in fine shape. Inflation will increase your income and costs at about the same rate. It only starts to hurt if you have a nest egg you want to spend on pandaren content. When everything is more expensive, your hard-earned pile of gold will be able to buy you less BoE gear, item upgrades, repairs, and pets than it will now.
Pets will be trainable and tradable in Mists of Pandaria. Particularly rare ones will likely see high increases in demand and price, even untrained. There's no way to know how much of a return you'll get from time spent training one of these things, but since there's no penalty for losing a match, I predict it won't be much per hour.
Your best bet is to pick a few rare drop mini-pets and watch the Auction House for them until launch, buying them if they're ever reasonably priced. You can research their average price on the Undermine Journal. The day the Pet Battle feature goes live, list everything you have, and don't lower your price for months. The fact that there are truly unfarmable drop rate pets combined with the generally higher level of gold most people will have will mean that you'll get your profit.
A few months into Cataclysm, the average price for some Wrath of the Lich King mats spiked, and they haven't come down yet. Enchanting mats, ore, herbs, and leather all went up. Much of the demand for these was linked to people leveling professions on new characters, but there are still some best-in-slot raiding enchants and very attractive bags made from Wrath mats.
Assuming that there are a few things like that in Mists, you'd be well served to keep at least some mats around in case they ever seriously spike. The main difficulty with stocking mats is that they take up a lot of room. Some of them, like Maelstrom Crystals, are quite dense, but the dust is very voluminous. In fact, if you're going to stockpile dust, ore, or herbs, you're best off dedicating a mail-bounce alt. Simply mail 12 stacks at a time to this alt, and 30 days later, they'll be returned unopened. It's a labor-intensive way to store things, but it's free and unlimited.
Speaking of maelstroms, remember that there's a price cap on them. As long as you have an enchanter with the ability to DE epics, you can get them for 146g from the Firelands reputation vendor. Of course, this is only going to work as long as Blizzard doesn't make a change like making all vendor-bought epics resistant to disenchanting.
You'll notice I left gems off this list as well; this is because of the new design for ilevel minimums. While Wrath gems are usable for any gear with a socket, Cataclysm gems are only good for 300+ ilevel gear. There may be a way to make a profit selling them in Mists, but I won't risk it.
As mentioned above, one issue with mats is that they generally take up a lot of room. Even assuming you're spending time every 59 days to re-mail your entire inventory, you still have to work pretty hard to store a small amount of gold (relative to even a modest auctioneer's average gold pile). The solution? Denser investments.
Rare mounts, vanity items, and TCG loot can often cost hundreds of thousands of gold. A few of these could easily reduce the amount of mailing you have to do. The key is to pick something that won't see a massive increase in availability next expansion -- rare raid rewards, for example. A Shadowmourne tchotchke costs more now than it did last expansion and should be even more in Mists. Everyone will have tons more money, and even fewer people will be building Shadowmournes. There is a very expensive and rare sea mount that will see less supply next expansion, and of course, Trading Card Game loot.
The main thing about high-ticket items is that they take much longer to sell. I wouldn't keep too much gold tied up in them, or else you run the risk of having to sell for a lower amount that you otherwise would have if you need your money quickly.
Also, TCG loot is not like in-game rare items in that the number available depends on the number of codes bought from the company that makes the cards. The supply tends to increase over time, and while you may get lucky and pick something that retains (or gains) value, you may get something that loses money.
Maximize your profits with advice from Gold Capped. Want to know the very best ways to earn 10,000 gold? Top gold making strategies for auctioneers? How about how to reach 1 million gold -- or how one player got there and then gave it all away? Fox and Basil are taking your questions at email@example.com and firstname.lastname@example.org.