Not to take things too off topic, but which characters were unnecessary in Avengers? Black Widow earned her place on the team after her involvement with IM2's story. Hawkeye was handled poorly but is a natural choice to include in BW is in. Then the only other major good guys were Fury who was VERY necessary, Coulson who was just about as necessary, and Hill who is a big name for fans and Fury's right hand so she deserved her place as well. Then there's only Loki and the scientist who had any real prominence but both had significant roles in the plot. So I don't see where your complaint is coming from.
Anyway, back to the discussion at hand, I used to be one of those people who insisted on the original seven and would not have it any other way. But after some time, I've come to support the five character lineup instead with the trinity + GL and Flash. This story, as we all have been saying from the beginning, will be responsible for introducing Wondy and Flash to the GA, but even more than that it needs to give them a reason to care about them and want to come back to see a solo movie down the road. So, they'll need as much screen time as they can muster.
Throwing in Martian Manhunter would make the breathing room tighter for introducing characters and giving them worth. If they save him for a sequel, they can even make the story center around him and have White Martians as villains. But if they must have him in the first movie, he shouldn't be an active, fighting member of the team.
I had an idea a little while back about how they could handle that very idea. Introduce an Agent/Detective Jones who works under the government as a trusted operative on higher level cases. When the Earth is threatened by whatever poses the threat for the movie, he is assigned to keep tabs on the Justice League as they do their thing and becomes their liaison with the government. Towards the end of the movie, we get teased at him having special abilities and we see his eyes glow red or something. Then he makes appearances in the solo movies similar to Coulson in MCU and in the last solo before JL2, he is revealed to be an alien and the White Martians are teased.
But for now, the JL will do alright without him or Fishstick.