Great concept. As you can see by all of the responses it is something your community is really looking forward to.
I think some of the suggestions are going too far, such as lands or buildings for half price. However a 10-20% reduction in cost is certainly more viable. People have different priorities though and you will never be able to cater for everyone.
Therefore, based on my own requirements, I would like to suggest the following for Black Friday;
1. Limited or preferably unlimited mithril sales.
2. Limited Mithril for sale in Oracle for real money, similar to Crystals.
3. Ability within Mage to sell unwanted equipment, inferno and aqua.
4. 2-3X gold on selected but good variety of EBs.
5. Increased drop rate for all items for the day.
Many thanks for for consideration.