The items you mentioned (door locks, thermostats, doorbells) all work the way I suggested. They require a central monitoring system/network that has the connectivity required to link a device to an app. That is not something easy or inexpensive to implement. Ensuring the communication is secure is even more work and cost.
That said, Roku has now branched into home monitoring and smart devices. It's possible that in the future they could provide a method to link a Roku player/TV to the home monitoring system. Since they are moving in that direction, it's certainly possible they could add it.
The problem with controlling a Roku player/TV is that there's no way for the person using the app to see what's happening on the screen. I'm not trying to discourage you, but advising some of the potential issues with controlling a Roku remotely. It would be extremely difficult to feed the screen image back to the app screen, especially leaving any room to both see the screen and the controls. You would have to rely on talking with a person that can see the screen, and depending on that person it might be difficult to work it out. I know how hard it would be to do it with my Mom, who is 88 and not tech-capable at all. Which is actually a bit surprising since she was a computer operator for Ma Bell back in the '60s-70s when they first installed IBM mainframes.
Let's get RokuDanny-R linked in to provide a conduit to the Roku main office and see if he can find out if this might be something that is being considered. It is a valuable idea to put forward. But I will warn you that Roku seldom/never comments on future developments, so while we can get our ideas and opinions to the company, just don't be surprised that you don't hear anything back.