Answering this question is pretty straightforward, and most posts in this thread have it partially right. The complete package would look like this:
Ironman. Easy to program: only soft saves, no hard saves.
Hard, not Lunatic. Trouble with the design of Lunatic is that it up the challenge by buffing enemy statistics to Herculean proportions. This is uninteresting, since it reduces strategy to stats vs. stats. It's telling that Thracia, arguably the most strategically engaging game in the series, has the most statistically puny enemy units.
Bring back Lunatic+ skills. For all of its faults, Luna+'s randomized skills made for a fresh approach. They were a dreadful fit on the buffed Lunatic enemies, however, and their random nature encouraged resetting to get optimal skill sets. These issues would no longer be a problem. Randomized skills on smartly-designed Hard enemies fits, and built-in Ironman precludes the possibility of save scumming.
Streamlined. No world map, no visiting shops outside the main story, no "merchant" shops a la FE8, no DLC, no renown perks.
The return of Fatigue and minimum deployment. Rankings don't appeal to everyone, but they effectively discourage low-manning (which absolutely ruined Awakening). There is another way to achieve those same goals: Fatigue discourages overuse and minimum deployment demands using many units. Unlike ranked play, this is forced, not optional.
Rewards. Because difficulty is high and death probable, reward players who brave the challenge with some unique content. Give players a new map or two. Give players an additional phase of the final boss. Create new dialogue where characters grapple over the loss of fallen comrades. These rewards don't have to be huge, just something to encourage a playthrough beyond mere difficulty.
tl;dr streamlined ironman Hard+ with fatigue, minimum deployment, and extra content
Can't imagine any veteran player would be dissatisfied with this mode!