I don't know if I'd want to make it hardcore, Feed the Beast is a little too expansive for permadeath or extensive deathbans. I think there's nothing wrong with most Mystcraft features but because of how Minecraft loads worlds, every loaded Age would be akin to one very large chunk loader.
It all depends on how big the community should be. I'd like to keep it to a few players, because large player counts trivialize some things as well.
So it seems so far:
Limit Mystcraft (dense ores, storm-inducing symbols, maybe accelerated).
Consider huge biomes.
Nerf Netherrack lava energy to be perhaps somewhere between biomass and biofuel? That way it'd be a plus to use Mass to power a crucible but even more beneficial to convert it to Fuel instead. Omicron suggests 16k cost.
Nerf solid fuel boilers, perhaps?
How do you guys feel about the solar panels? The highest tier solar panel gives 512EU/t during the day and 64EU/t at night. That averages to 288EU/t, which is about the same as a well-designed nuclear reactor give or take some, but NEVER requires fuel. I'd have to disable the Eternal Day symbol as well otherwise players could use eternal day with the IDSU. The more people skip night by sleeping, the faster this average approaches 512EU/t.
Realistically, the deal with solar panels in the real world is that, they provide effectively infinite energy (the sun is going to outlast the panels in any realistic world situation) but they take up SO MUCH LAND. Huge flat areas of solar panels required to power anything large-scale. To let a single-block generator bring in the power you expect from nuclear fission for no fuel whatsoever is fairly OP, even if it has a significant crafting cost.
Advanced solar panels were originally designed to prevent the server lag caused by bunches of individual solar panels, but they've scaled too high in my opinion.
