Initiate command in another shell session?

The question occurred to me whether it would be possible to remotely
execute a command as if started from another shell session *in* that
shell session.

My first thought was that it might be an undesirable property (if only
for "security reasons", or more likely, generally).

What I can do is
* sending some data to another shell session, say,
    ls > /dev/pts/22
* get the output of a command executed on another host, say,
    ssh somehost ls
* send the output of a remote execution to another shell, say,
    ssh somehost ls > /dev/pts/22

Note that the latter will not operate in the shell context of the shell
session that is running on somehost with /dev/pts/22.

I guess the answer to my question is simply "no", but I'd like to have
a confirmation (and possibly more specific rationales or remarks).

Janis

PS: On an old SunOS 4 SPARC system I was (without privileges) able to
even change the remote users system characteristics (like mouse speed),
but I suppose that must be considered a security issue on that platform.