Return of the Lich King was trademarked by Blizzard a while ago and redditor PenguinJ2 posted some solid pointers as to why it could be the next Hearthstone expansion.
1: Return of the Lich King was trademarked four months ago by Blizzard.
In a reddit thread four months ago, someone noticed that blizzard trademarked Return of the Lich King. You might think that this could be a WoW expansion, however speculation in the comments of that thread think it's a hearthstone expansion. It doesn't sound right to be a WoW expansion, and would be weird for them to bring back the Lich King now. It is also quite unlikely that it is for heroes of the storm, as they just had a big event (hots 2.0 and hanumura showdown). The name sounds very "hearthstoney".
Just this alone is unlikely the next expansion for Hearthstone will be Return of the Lich King, however the next two points make this a lot more likely.
2: returnofthelichking.com was parked by GoDaddy four months ago.
Blizzard have done this for all of their expansions since Old Gods at least (idk whether they did that further back), with the site GoDaddy. Again, this alone makes it unlikely that the next expansion is Return of the Lich King, and it could have just been done for a different game. But what is interesting is that this was parked about the time that Return of the Lich King was trademarked! This seems the perfect time for a hearthstone expansion, as they generally are announced about a month from now and released about two months from now. It could have just been a fan who parked the site to try to earn money off blizzard, perhaps, but I doubt this as blizzard would have parked the site as they trademarked the names.
3: The last two ranked card backs were the places Arthas visited in order.
Last week I saw this reddit thread which sparked me to make this post. The other two points above aren't enough evidence alone that the next expansion will be "Return of the Lich King", however I think this makes it much more likely, as Arthas was the Lich King. As the thread states, if the next card back is something to do with Northrend this is pretty much confirmed, however I didn't want to wait until then as next month there will be an update (as there is no ranked card back), that will most likely be the pre-order of the next expansion. This is very unlikely to be a coincidence along with the other two points.
4. Datamined Icecrown Image
Someone pointed out that this image datamined by hearthpwn from the last patch might have something do to with it. Icecrown hype? Or something from brawl?