Flash sales are awesome, we can agree on that I think. A short time to get a hold of some rarer accessories that seldom show up in the game otherwise. However, in the past couple of years, there have been a grand total of 2 flash sales and that's it, one on each Cyber Monday in November. I feel that's a bit scarce for them and some popular accessories are becoming quite hard to come by.

I would say, let's make flash sales a liiiittle more common. I'm not saying one every two weeks or even every month. I'd be pretty happy to see one spontaneously show up every 3-4 months personally. It'd be fun to log on and surprise! Flash sale for the next 24 hours with a change in available items every half hour. I'd argue for half an hour because that allows for some of the item to get into the market but not everyone and their brother can get a dozen.

There's also a ton more items that a lot of people would like to see in flash sales along with the old favorites like pris wolver tails, pris valks, dragon wings, etc. Some examples of the rares we would like to see are from old promos such as the first generation (white, red, blue) chef coats and hats, old promo auras like dreadful and rose, lovely and verdant whiskers, tomes of rage, slippers, gun pup helms, and various sprite reskins like gun pup drakon, snarblepup maskeraith, divine and shadow drakons, freeze and volcanic maskeraith, spiralhorn maskeraith, spiraltail drakon, etc.

Now, I wouldn't say that things like the gun pup drakon and the various slipper colors should be cheap, heavens no, they were a >=2% item during their respective promos for a reason! But some of those items are not just incredibly hard to come by but impossible to come by, if any unbound tickets/unopened pods even exist in the game anymore. It'd be cool to see them show up on players every now and again, even if you have to shell out 20k E for a pair of slippers.

Edit:
Fangel had a good point that while flash sales do introduce a ton of some rare items provided you're online at the right time, it doesn't help the energy market all that much. It tends to make energy spike in price when it's already too high as it is.

A good solution, in my opinion, is to do flash sales and Auction House sales, perhaps interchangeably depending on energy prices at that current time. Flash sales would cause energy prices to go up due to energy being in demand. AH sales are an excellent crown sink and bring energy prices down due to less crowns being in the economy (let's face it - the really big ticket items tend to go for a huge amount. Lookin' at you, prismatic scarf!) as well as crowns being in demand.

Another idea for AH sales would be to perhaps AH multiple of a certain extremely rare item. For example, 5 rose auras in separate auction listings on featured. Some select few items are so incredibly rare that you seldom see them even on players, no less in item form so I think this might help a few more extremely rare items come into the economy while not boosting energy prices like flash sales can. One problem with this idea is that you'd have way more pages on featured than usual and if they're all listed at the same time, will all end at the same time. This could be countered by rolling out the listings slowly so the end times are spread out.