There was a phone not too long that was the same style as blackberry, in terms of keyboard, but was not a smartphone in any way (as opposed to a regular blackberry). It was pretty standard to see it in non-smartphone settings and nobody gave a second look. i personally find the slide keyboards easier (im still using a 3g sprint side phone) but they break around every 2 years in my experience.
Another point is that i feel the market for flip phones, even though they want a full keyboard, do not want the other possible features discussed above like email and spotify, even google maps. It seems like software is less of challenge than hardware, so maybe make two options, if that is possible, to access different markets.
One more point is that i think there should be a poll with all the different options, to get a better idea of popular opinion