It would be great if it was chock full of the beta monsters that were removed, because they were "too difficult" to fight against. Along with a couple of other new ones to make a variety. Creatures like the tortodrone, slooms, vog cub. Creatures and maps that are totally alien to what we experience everyday. Sure the very old players would know how to beat them, but they're a dying breed and you could always add changes to them. This whole area would look totally different. It would have elavators that are just pads of pure light that would take you down instead of elavators made from the Spiral order and machinery and beasts that didn't exist eons ago with powers unheard of pulsing with small doses of power from what powers the core.

Maybe add in new status effects that knights would have resistance against. Something like "foreshadowment" which makes black blots appear on the screen at random for a short time so it's hard to see what's going on, especially around turret towers shooting projectiles you have to avoid. Another could be "corrosion", a water themed affect, where either the your sheild or one of your weapons becomes useless for a set time. Maybe even add a new type of damage that would be critical since no armor would protect against it. Something like "cosmic" with a blue attack icon (to finally match all the colors from what you see on the minerals.)

And of course something like this would need a killer boss at the end. I can just imagine the whole set of missions devoted to reaching the core after vangaurd and beating the shadow lairs. The storyline would be something like the gremlins decided to help knights in an awkward alliance to open up the core. There would be missions where knights must retreive certain parts and equipment from unkown parts of the clockwork where only the gremlins know of. When they finally open it up the gremlins turn on the knights and get a headstart to explore the core which begins a race between gremlins and knights to obtain the ultimate source of power found in the center of the core.

I can imagine right when your squad is about to reach the end, King Tinkinzar comes up with a beaten up squad of gremlins, snatches it before your eyes and begins to brag about how he'll (mis)use the power. Once he's done bragging the whole scene looks like there will begin a battle between both beaten up teams when suddenly I gigantic machine, something like the core's "ultimate gaurdian" attacks the gremlins with single blast which sends them out of the picture and then you fight it after the short 10 second cutscene of its appearance.

The last things I'd like to add is that it would be scary tough with no heart restorations before the boss fight AND if all knights fall in the middle of the battle this "gaurdian" will simply kick out everyone before there is a revive. (You can only revive as long as someone is still on their feet.) The only advantage that should be given for exploring such a deadly place is to grant a squad to have more than just the 4 person limit, but actually have an 8 member squad that can meet up right outside the core. Something like a full lockdown team to give it a go in what would ordinarily be impossible to just one person. It would at least be requirement that you can't go down without a full squad. Whatever players that aren't there to fill the party will be given over to comp knights to accompany you. If a player leaves while in the core, his/her place won't be replaced with a knight since you passed the checkpoint. (You'd probably be given 7 comp knights, but who would want to do that just to go it alone? You'd be better off with a team.)

I think the center of the core should be something so extremely hard it gives the well-experienced players a scare, but the ultimate prize at the end to be worth it. Something so hard and advanced that it would probably takes months after it comes out for some lucky party to make it. people would play it regardless of how tough it is or not, because no one would know what would be beyond it. It the curiosity for to discover the unkown and be the first one to claim it out from everyone else. It would be the climax of everything where extremes from one end meet extremes on the other end and collide into something unimaginable.

And when the core is finally opened, the game could still go forward by exploring the inner depthes of different planets, or what's on the surface of this one, but that's a conversation for a different thread.

(Sorry for the long read. I just felt like pouring my brain out.)