I would like to see some mechanical benefit. My thought would be to allow a different character to play the scenario at regular difficulty if you complete AND survive the mission at hard difficulty. This would be particularly useful for future faction ending scenarios.
I don't like the idea of hard difficulty for lower tier groups. Generally they have less of everything and can get in too deep very quickly. If they have the levels, they will play up anyhow and get a material benefit for the risk.
I would like the option to opt out of hard mode midway through. Everybody bites off more than they can chew sometimes. There would not be an opt in though.
I would most like to see 6 player mode and/or hard mode published for season 0-3 scenarios. Most of these are in serious need of rewrites, even if only tactics and spell selection.