"Topic" is actually quite correct for this. In philosophical discourse, the kind of arguments Aristotle is discussing in this work can properly be called "topics". You are quite right, however, that "topic" in modern English comes with another common meaning attached (an issue or subject) and that this could cause confusion among some readers.

The solution that is often adopted is to use the Greek term "topos", plural "topoi". It is standard in the academic study of literature for a commonplace theme or formula (known as a "tópico" in Spain), and is very commonly used by professional philosophers when discussing this subject in Aristotle, as I think the following results illustrate:

https://www.google.es/search?num=100&site=&source=hp&q=Arist...

The advantage, of course, is that it's accurate but no one will mistake its meaning, as could happen with the English term "topics".

So what about endoxa? It's a very closely related concept, and is certainly part of Aristotle's terminology, but it's not exactly the same thing, and in this field small differences matter. Endoxa are commonly held opinions. Topics or topoi are arguments resting on such opinions. Endoxa are premises; topoi are arguments.

Here's an expression of this in Spanish:

"Por todo ello parece acertado decir, con Bornscheuer, que en Aristóteles topoi y endoxa designan diferentes aspectos de la misma cosa: el concepto de topos remite al carácter instrumental de todo punto de vista argumentativo generalmente aplicable, mientras que el concepto de endoxa se refiere al momento del reconocimiento social general de tales puntos de partida."
http://books.google.es/books?id=wjwkQHrRIMMC&pg=PA59&lpg=PA5...

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Note added at 3 hrs (2014-07-07 07:31:55 GMT)
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And here is valuable confirmation from Robin Smith, Professor of Philosophy at Texas A&M and author of the standard modern commentary on Aristotle's Topics (Oxford: Clarendon, 1997):

"These collections of endoxa will be useful for telling us whether our opponent will accept any given premiss, but they will not of themselves tell us which premisses to put forward. That is the job of a second component of the dialectical art, one that is far more significant for the history of philosophy and logic: the collection of topoi, 'places' or 'locations'. [...]
What, then, are these 'locations', these topoi, which give the Topics its name? Unfortunately, the Topics itself does not tell us in so many words. However, Aristotle does say that the internal Books II-VII of the Topics consist of topoi".
http://www.amazon.co.uk/Topics-Books-VIII-Clarendon-Aristotl...

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