I honestly think the goal of tragedy for him is catharsis. And the role of the plot is a story to follow in a logical order.
Everything is contentious in the interpretation of a text, and a philosophical text is not exception. With that in mind, I offer the following :
Goal of tragedy
Aristotle sees tragedy as a goal-directed system. The goal is catharsis. He disassembles tragedy in order to see how each part functions to promote that goal. Precisely because each part does individually contribute to the whole, no one part is absolutely necessary. (Catherine Lord, 'Tragedy without Character: Poetics VI. 1450a24', The Journal of Aesthetics and Art Criticism, Vol. 28, No. 1 (Autumn, 1969), pp. 55-62: 57.)
We'll note the parts in a minute. First, how are we to understand 'catharsis'? Aristotle has in mind a katharsis pathematon (Poetics, 6. 1449b28). Tragedy arouses pity and fear but in such a way as to achieve the catharsis, the curing or purging of such emotions by exhibiting them as inseparable from human life and not as unfortunate experiences that might happen merely to ourselves and to chance others. By setting them in this universal frame, tragedy also puts them in perspective and helps to regulate our reaction to them and to understand them better in the lives of ourselves and others.
Secondly, a statement has been quoted above, and I have some difficulty with it :
"The word catharsis drops out of the Poetics because the word wonder, to rhaumaston, replaces it... in chapters 24 and 25, where he singles out wonder as the aim of the poetic art itself, into which the aim of tragedy in particular merges."
Poetics, 24. 1460a12 refers to to thaumaston (wonder) and Aristotle does concede that 'The element of the wonderful is required in Tragedy' (S.H. Butcher, The Poetics of Aristotle, London: Macmillan, 4th ed., 1936: 95.) I question whether and where wonder is installed as 'the aim of the poetic art' under which tragedy is subsumed and as a consequence catharsis is demoted.
Place of plot
Tragedy has six parts (Poetics, 6.1450a :
When we find Aristotle expressly giving the pride of place to plot among the six parts of Tragedy - Plot, Character, Thought, Diction, Spectacle, and Music [song, melody: GT] - we are instantly perplexed, if not outraged. Even those of us who may be sympathetic to Aristotle on some fairly general ground are strongly tempted to suppose that what Aristotle means by plot, mythos in some places, logos in others, cannot possibly be that crude thing, namely, story and action. He must mean something more subtle than that. Those of us who are not prepared to give Aristotle the benefit of the doubt will insist that he does, indeed, mean the crude thing and that he is insensitive to the finer features of the literary art.... [But] Aristotle does mean that plot, story, action, the crude thing, is primary. (Lord: 55.)
Plot (muthos, sometimes logos)
Song/ melody (melos)
Hold on : thinking ? dianoia ? Explain !
Under dianoia are included the intellectual reflexions of the speaker; the proof of his own statements, the disproof of those of his opponents, his general maxims concerning life and conduct, as elicited by the action and forming part of the train of reasoning. (S.H. Butcher, Aristotle's Theory of Poetry and Fine Art, London: Macmillan, 1902: 27-8)
Rationale of the primacy of plot
The Plot ... is the first principle, and, as it were, the soul of a tragedy (Poetics, 6.1450a.37; Butcher: 27-8).
How so ?
I think Aristotle's basic idea is that the plot alone can fasten the whole together. Its unity and coherence of action - the passing of good fortune into catastrophe, of success into failure, of love into hate, of pride into shame, &c. - is the sole string on which the other elements - character, diction, thought, spectacle and song or melody - can be threaded. Without plot, tragedy has no backbone of structure, no focus, only episodic or incidental points of interest.
Connection with catharsis
The clearest link between the primacy of the plot and the goal of catharsis is that the goal will not and cannot be achieved unless the tragedy has the unity of action that enables a catharsis of our emotions - and only the plot can provide this unity.
Whether this is a sound, the soundest or even a plausible analysis of the relation of plot to tragedy, I venture no opinion. I have simply tried to make the best sense, consistent with the text, of Aristotle's stress on the primacy of plot.