That's what the week limit is for. If someone passes their 7 day period, they're skipped and the next person takes up. Perhaps, they have 2 days to pick up the file, and if they fail to pick up then they're skipped quickly. If they've picked up the file and don't drop it within 7 days of the last drop, then they're skipped. Skipped players may go to the back of the line, as can recurring players I suppose.
Make caves visible, and do a lot of them. Their positioning is such that you can easily avoid them, but it can be very fun to do an embark on one. Most importantly, it provides a way to get underground without digging, which can make for some highly unusual fortresses. I could imagine the sort of lore a fortress would generate being founded on a cave anyways.
Aquifers could be a player's preference, because it's possible to disable them, embark, and re-enable them, in case the next player wants an aquifer. Some just can't handle them, and that's fine!
Also, I'll point out that my computer can semi-reliably handle 16x16 embarks, or at the very least can handle 8x8 or 8x16. So I can go spelunking for upright fun in case someone wants a fort on one. I can't promise to find you an upright that's by a river and a cave with clay and shallow metals, but I can promise to find you an upright in a specified biome type. To get an upright, you need at least 2 cavern layers, from my experience.
I'd suggest a medium-shallow world, help with the FPS a little there, with slightly above-average mineral richness (ie, lower scarcity). 2 Caverns, again for FPS reasons, you don't lose anything as the caverns are compacted into fewer layers.
Should we do any mods? For construction purposes, I'd vote to include a display case mod to show off the fun artifacts and whatnot. Might also add a specified stone type and a method to obtain it for free. This stone would melt at the temperature of a magma-warmed floor, so you could deconstruct a display case by running magma under it, allowing for a variety of levers or pressure plates to be used to give the item. If you're unable to obtain magma in a suitable time for the display case deconstruction, DFHack may be utilized to apply a small amount of magma to the area, for the purpose of making an interesting dungeon. Magma cannot be hacked in to make a trap, the ONLY legit hacking in would be for this specific use.
What graphic pack, if any?