Imagine my goal is to calculate the maximum number of users supported to get an average transaction response of 3s?
Or imagine that the goal includes more complex rules as a ruleset, such as increase the number of threads until the response time averages 5s or the slowest page exceeds 10s or this particular page averages more than 5s.
Is there any such thing for JMeter, even as a plugin?
As far as I know you can use a jmeter plugins' Auto Stop http://jmeter-plugins.org/wiki/AutoStop/
I never used it by myself, but I see that you can configure it to stop on given response time (for a given period of time). And it looks the easiest and straight forward way to do it. You just have to download proper package (I found it in Extras Set)
-EDIT-
Oh, I forgot to write that to create test case that will increase the load gradually you can use Stepping Thread Group from jMeter Plugins as well (http://jmeter-plugins.org/wiki/SteppingThreadGroup/). Pretty straight forward and, when grouped with Auto-Stop, gives you exactly what you are looking for - larger amount of threads in periods of time to find desired load.