Imperfective aspect or Future tense?

You need to distinguish between tense, aspect and time reference. For instance, the English present continuous combines a present time reference with an imperfective aspect. However, it is often used to denote future time reference as well in some contexts. E.g. "We're leaving at 8 sharp tomorrow morning."

What you need to do is a functional analysis of the whole system of tense, aspect and mood. I tried to something like that for Czech, here: http://www.slideshare.net/bohemicus/presenting-aspect-as-tense

The whole system of labels for tenses and aspects is really a mixture of tradition and some general principles but their definitions are not reliable to predict actual uses across languages. For instance, in the above presentation I showed that you could easily reimagine a Czech system without aspect or the English system with only three tenses and three aspects. These things are just more or less arbitrary labels for ways in which languages code reference to time, perspective on action progress vs. completeness, repetition, mood, etc.

That said, I find the labels 'tense' and 'aspect' useful shortcuts when describing these complex phenomena - particularly in broadly related languages - but they're just that.