Alright so basically after reading all of your all's input, along with talking with mbl about the subject, I have a couple answers for everyone, if not exact answers, then they're pretty close.
Basically we need to look at mods that are fun and helpful, but don't ruin the server with lag. This means nothing that will generate game-destroying biomes, along with a limitation on who can do what. Chunkloaders can be a problem, and will most likely be limited to a (16) chunk area, or we could settle with the lesser form such as Cod said, but with no limitations. This will allow players to keep their quarries running, along with their farms and whatever else they enjoy. Quarries will also need to be limited to a smaller area. I was thinking possibly no larger than 100x100, or something of that nature. Last map people would have extremely large quarries set up that created constant lag, and this is unfortunate to those who have crap FPS in the first place.
Along with limitations dealing with lag, we also need to look at the grief/protection part of mods. For example, Thaumcraft has items in it that could be used for major grief. Now don't get me wrong, Thaumcraft is a wonderul mod that'll almost certainly be used: however there are other things are there not quite as safe. This includes, but not limited to TNT and explosives, IndustrialCraft and BuildCraft tools and machinery, and other weaponry mods. Rules will need to be strictly inforced on this matter, and if anyone would like to suggest and/or find a mod that can (lag-free, area-prot free) protect blocks and chests, feel free to do so.
Ranks and Whatnot
Basically ranks are possible, but aren't necessary to the running of the server. Two suggestions would be, but again not limited to these ranks:
Guest, Member, Op, Trustee, Owner
Default, OP, Trustee, Owner
The top suggestion gives regulars a colorful name, whereas the bottom suggestion is just the common ranks. Honestly, permissions wouldn't really be needed with a ModPack, because there would be no restrictions that could be "unlockable", for most restrictions will be due to the reduction of lag.
ModPacks and Customs
If you all want to create a modpack for us, that would be terrific. If you wish, you can create one based off what you think the server should have, and then show it to mbl; you can also talk with mbl beforehand making the pack, and you'll then know what mods are required, then can go off of that. We'd like to use one that is already created, however if you look at most: almost all have many many mods and would create saddening drops of FPS.
To what I said earlier,
Based off of mbl's reaction and comments to what I said, the Enderdragon and Wither fights would be... Difficult. Just because non-forum players could early kill the dragon, and the Wither can cause some lag. Therefor, those events (probably) won't happen.
PvP can still happen, but maybe with a lesser tier armour, not unbreakable.
If PvP happens, it would be something with all iron-leveled armour, along with tools of your choice. This is just because some armour is a hella lot more protective than some, and can be unfair to players.
Building challenges are definitely a possibility. Building topics could be "Automatic Furnace-Filler", or who can build the best defense against something. Ideas are endless, and can be discussed at a later date.
If anyone else has questions, or mbl/Jonesy/Ondra if you see anything here I said differently than what you're thinking, go ahead and pop a comment. Of course that offer goes to you all, but those people are the ones who I've shared ideas with. (Well, besides Ondra.)