XiaoMi has some very popular products in China, but part of that popularity is due to their own artificial scarcity strategy of releasing new models only for a short period of time. But as any company that wants to grow bigger, that strategy can only work for so long if it's the only strategy employed by a company.
That doesn't necessarily mean they should just start selling all of its products the normal way, though, which would also imply a constant barrage of advertising throughout the year, to get people to keep buying their phones, even the ones that are close to a year old. They could, however, use a mixed strategy of selling some phones throughout the year, and while continuing to launch some a "Limited Edition" as they do now.
The "normal" phones could be some flagships that stay powerful throughout the year, and also some low-cost devices for the mass market. The Limited Edition phones could come with a more exotic hardware or software feature that would make people want to pay more for it. They could also sell only a certain amount, to make it clear from the beginning that the product is scarce, and if they want to get it, they should buy it before it's gone.
This way XiaoMi and some of its phones could still remain "hot", while letting other phones that they'd be selling through out the year to piggy back on that popularity and branding, and being able to sell a lot more of them. XiaoMi has dreams of going global, so they won't be able to always do "flash sales", considering phones arrive on a different schedule in different countries.
Before they even go global, though, XiaoMi should ensure they have a very strong and entrenched position in its home market, China, because that's also going to be their biggest market for the time being. It wouldn't be very wise to spread themselves too thin, and risk losing the local Chinese market, especially when so many companies are trying to "win" in China.