The only possible link all the champs is that there is a deception theme common among many of them, but that inherently leaves Hecarim out.
The more interesting thing to think about is what these shadows on champion pages mean. It could resemble a new champion release, but champion releases don't usually come out that quickly, and I personally wouldn't expect any new champion releases after the back-to-back releases of Gnar and Azir, especially considering Azir had a super rocky release absolutely riddled with bugs.
In addition, the most recognizable possible theme of a new champion from what we are currently seeing suggests a shadow-based champ, but that kind of champ has been done already with Nocturne and Zed, so that kind of champion seems very unlikely.
However, a more likely prospect is that it is for the Harrowing. The shadows fit perfectly with the theme of it, and even though LeBlanc is the only one of those champs getting a Harrowing skin and only a couple of them already have a Harrowing skin, the shadows don't really need to be on any specific champs to really have the shadow theme. In addition, it may be a sign that Riot wants to do something special for Harrowing this year, as Underworld Wukong and Ravenborn LeBlanc are already set to release during this patch, Reaper Soraka is already on the PBE, and Twisted Treeline Hexakill was in testing on the PBE last cycle.
The shadows are most likely to signal the Harrowing event.