The features that you will read below are under heavily discussions and are for the most part inspired by the hack and hear wishlist

• better signal processing
• seemless compatibility with the hearing aid and peripheral hardware
• avaibility of replacement parts
• ability of updating the software
• security of the system (unwilling modification of the parameters)
• long bettery life and standardized size
• adaptation of the hearing aid in different conditions (machine learning)
• calibration and modification of the paramaters by the user
• waterproof
• affordable price of the hardware

Addendum

After following an online course given by the team of Manchester, I think the following criteria are relevant (especially for poor countries):

• can recharge without electricity (can recharge itself by movement or solar panels or by a system that can generate electricity without being pluged-in to an electric network such as a dynamo).
• must be really cheap (not much more than a Raspberry PI)
• robust to temperatures
• robust to dust (correlated to be waterproof)
• all parts can be changes in case of a malfunction (easy hackable system)