For people who see privacy as the main concern, their biggest requirement will just be that the service is reliable and actually secure. Your job will be to convince them to trust you. Based on what you've written, I would say you have a decent chance of achieving this. If you can demonstrate that you are the most privacy-friendly service currently available, they'll be willing to tolerate a lot of inconvenience.
For those for whom it isn't, they will typically have a very different set of concerns. In this case the inability to send voice messages, videos or most things that the first group would see as non-essential will probably be a deal-breaker over time and will make them switch back to another service out of accumulated frustration. At the very least you will need support for image files, gifs, and generally making it easy to forward messages, links, and documents. However, this is the least of your problems as the main obstacle for you is that the network effect of actually having people they know on the service will be what they think of most and what will orient their choices. To get started, you then have to forget about asking what features are the necessary minimum and focus on what you bring to the table that is surprising enough to kickstart your own network effect. If you look at Whatsapp and then Snapchat and then Telegram, they all had that. In your case you'll most probably have to out-Telegram Telegram somehow.
I'm basing all this on what I've observed over the years in terms of what people tend to actually do with such apps. I'm firmly in the second camp. I even use WeChat, which is a horror show in terms of privacy, because that's what many of the people I've met use and since they live in China it's like trying to convince a Westerner to give up Facebook. Doable, but almost certainly an uphill battle requiring them to be predisposed to the idea in the first place.
For those for whom it isn't, they will typically have a very different set of concerns. In this case the inability to send voice messages, videos or most things that the first group would see as non-essential will probably be a deal-breaker over time and will make them switch back to another service out of accumulated frustration. At the very least you will need support for image files, gifs, and generally making it easy to forward messages, links, and documents. However, this is the least of your problems as the main obstacle for you is that the network effect of actually having people they know on the service will be what they think of most and what will orient their choices. To get started, you then have to forget about asking what features are the necessary minimum and focus on what you bring to the table that is surprising enough to kickstart your own network effect. If you look at Whatsapp and then Snapchat and then Telegram, they all had that. In your case you'll most probably have to out-Telegram Telegram somehow.
I'm basing all this on what I've observed over the years in terms of what people tend to actually do with such apps. I'm firmly in the second camp. I even use WeChat, which is a horror show in terms of privacy, because that's what many of the people I've met use and since they live in China it's like trying to convince a Westerner to give up Facebook. Doable, but almost certainly an uphill battle requiring them to be predisposed to the idea in the first place.