Tahm Kench shouldn't be able to carry an ally into enemy fountain.
Tahm's W is moderately trollable but overall I haven't found it to be much of an issue, except in the case of a troll Tahm dragging an ally into enemy fountain when pushed up to nexus, killing them both. There needs to be something to keep him from walking into there with an ally in tow because it's basically instant death. Not to mention that in some cases doing this could allow a team near defeat to make a last push and end up winning the game because of the troll. I know Riot lets some troll moves stay because most of the time people us the abilities in responsible ways but in this case all it would take is to make him unable to entor fountain with an ally caught in his W, it wouldn't take away from any proper uses of the ability.