Well, the easy answer is to say that the stem isn't imperfective. What marks imperfective aspect in the regular thematic paradigm is the lack of marking--i.e. the lack of the sigma or kappa. So if the sigma is there, then its perfective. If the sigma is not there then its imperfective.
It sounds like you're ending up in the right place. Future forms, cross-linguistically, tend toward muddledness. This is true even in English, where on the one hand the form clearly denotes Future tense in its most prototypical usage, but on the other hand it actually eixsts functionally and paradigmatically in the Modal system. In terms of the actual formal features, English is a language with a past vs. non-past verbal system, even though we native speakers clearly (and rightly) recognize having past, "present" and future tenses.
In terms of Greek, there are two views of the original of the future. It's either derived from the Proto-Indo-European desiderative suffix *-s which looked identical to the current aorist and subjunctive forms. Or its derived from the aorist or aorist subjunctive. Despite the views of some, there is no consensus on the question, though many of the desiderative people like to think there is. Jo Willmott's The Moods of Homeric Greek (2005) gives a recent argument against the desiderative hypothesis. Personally, I'm inclined toward David Lightfoot's observation in his 1975 monograph Natural Logic and the Greek Moods, when he said that there is really no clear way of choosing between the two historical options.
But all of that, in a sense, is water under the bridge. For the Koine period, what really matter is how the native speakers comprehended the form who, when learning the language as toddlers, don't get any of that history explained to them. The system is recreated every generation. Ontogeny does indeed recapitulate phylogeny, but which phylogeny? That is to say, the process of language learning parallel the process of language evolution, but there are multiple paths of grammaticalization to get to a Future tense: Modals, Perfectives, lexical items denoting movement, temporal adverbs, etc.
All of this connects closely to the question tense in relationship to mood. We like to make a big deal of the fact that there's no past tense outside the indicative in Greek, while ignoring the fact that the past tense isn't possible in non-indicative moods for English either. We cannot say "You should went to the store for milk" any more than a Greek could." But like Greek, we can say "You will go to the store for milk" and without a context not be sure whether the sentence is referentially future referring (go there tomorrow), imperative (commanded to go there), or modal (obligated to go there).
I'd say that actually contributed a great deal, in very clear form. Thank you, Mike.