Russian press quotes Vladimir Degtyar, the director of the Makeyev Design Bureau, as saying that his bureau is working on a new ballistic missile. Apparently the contract announcement has been posted at zakupki.gov.ru, but I cannot find it at the moment. In any event, Degtyar's words suggest that its a new missile, different from Sarmat:
Today, we work on several contracts for the defense ministry, developing land-based and sea-based ballistic missiles. It's the new heavy ballistic missile Sarmat [...] and development [опытно-конструкторские разработки] of a new prospective machine [=ballistic missile]
The Izvestia author speculates that this might be a new SLBM, but the evidence he gives is extremely thin, so it's in the "not confirmed" category for the moment. However, it is quite likely that it is indeed a new SLBM - there is hardly a place for another land-based missile. Moreover, I would not be surprised at all if Makeyev works on a new liquid-fuel SLBM. I hope we will find out.