Every character in a film must have a "dramatic purpose" to justify their existence. On the surface it would appear that Tank's dramatic purpose is twofold. First, he is there as a lifeline for those in the Matrix, he warns them when agents have generated and guides them to exits. Second, he kills Cypher to save the rest of the crew , and surely you'd agree that The Matrix wouldn't be The Matrix without Trinity, Morpheus or Neo?
Beyond these obvious purposes is another, due to the fact that Tank is a "pure human". He is there to show Neo the world outside the Matrix, to explain this new reality from a different perspective than the Matrix "born" crew. Tank and his unfortunately deceased brother Dozer tell Neo about free humans. Tank is the one who tells Neo about the "promised land" of Zion from first hand experience.
Tank is an essential character in the sequels because he has already been introduced to the audience and has established his dramatic purpose. More pure humans with their own purposes will no doubt be introduced if the remaining crew travel to Zion. In our opinion Tank should have an expanded role by virtue of surviving the first film and since he is already known to Neo as a reliable ally.
Marcus Chong should continue to portray Tank for the same reason, he is already known to the audience. He brings depth and intensity to the character with an amazing range of facial expression and a magnetic presence. Of course, for the ladies, it doesn't hurt that he's pretty easy on the eyes too.
©2000 The We Want Tank Coalition