Is there a way to specify a sequence of messages every connection should send? Preferably even let it randomize it (want to load test a game with real input, not just set up the connection).
You can randomize input somewhat by using \{connection.id % 100} expressions right in the -m and within the files.
But yes, tcpkali can do simple randomizations, but does not support full-blown scripting. For that, consider using http://github.com/machinezone/mzbench or other alternatives.