"Do we want to order a banana cake?"
In English, there is a difference between "do we want to order a banana cake" and "do we want to order banana cake".
Wouldn't 我們想点一块香蕉蛋糕嗎 be a better translation than 我們想点香蕉蛋糕嗎? (assuming the current English source is intended)
Also, do we have to use 要，or does 想 work as well?
I didn't have 点 on my answer and it was rejected but i don't see what purpose it serves.
It means "order". In most situations (just like English) it would be understood without stating it. "Do we want to order cake" vs "Do we want cake". But I can envision some situations where adding it would avoid confusion: you're at home and say "do we want cake?" You might get the reply "I didn't know we have cake in the house"
I guess the question was written by someone from mainland China. In Hong Kong, where Cantonese is spoken, 点 is seldomly used. You are right, for all intents and purposes, 点 is colloquial. On the other hand, it is so prevalent. You better get used to it if you want to order some food in China.