In some varieties of English, including my own, [ʔ] is a very common allophone of [t] in certain positions, namely midvocalic and syllable-final. As it is an allophone in complementary distribution it has not previously been referred to as a phoneme in its own right.
But the fact that, at a supramorphemic level, a distinction can be (and is) made, for example the difference between the phrases "great ape" [ɡrɛɪʔɛɪp] and "grey tape" [ɡrɛɪtʰɛɪp], would imply that [ʔ] could veraciously be introduced as the seventh phonemic English plosive (in certain accents).
This is my own opinion; I haven't yet asked any other linguist but am interested to hear what the rest of you have to say about such an idea. (And I apologise for this not really being in the right category, I couldn't think which would be better than "language learning" out of the ones provided).