Huawei has trademarked the "MateX" name suggesting that an upcoming launch of a phone with this moniker could already be in the works, or at the very least that Huawei is planning on developing a device with this name and releasing it at some point. That said, just because Huawei trademarks the name MateX does not mean that it will actually end up using the name for any devices it does launch. What it does do, is ensure that it owns the rights to the name so no other companies can use it for any products they plan to sell.
According to the date listed in the screenshot below this is a rather new filing for Huawei and the EUIPO where the trademark was logged, as it was filed back on February 9 of this year. This may suggest that a device called the MateX is not exactly close to being finalized, but could still be a device that Huawei wants to release in the future. Huawei already offers the Mate 10 and Mate 10 Pro, so the MateX would not be a replacement name for the number ten which the company has already used to refer to the most recent iteration of the Mate smartphone series.
With that being the case it's unclear what the MateX could be, if anything. For the time being it's just a trademark that Huawei has now filed for and with the Mate 10 Pro having just been released, a Mate 11 isn't likely to be just around the corner, especially considering that this time of year tends to be when Huawei gears up to make announcements for its P series devices. Worth mentioning is that Huawei has not yet been granted this trademark by the EUIPO, though it's unlikely that it wouldn't be approved. Huawei is suspected to be announcing the next P series devices at an upcoming event in March according to the date the company teased on recently sent out press invites, though it's also possible it could end up announcing them at a later date. The Mate 10 Pro, the most recent device in the Mate lineup, went up for pre-order in the U.S. back in the beginning of February.