From what I read from the gencon coverage the 4th ed Tomb will be slightly different. It is going to be a modular ongoing plotline that fits in around an existing campaign - you don't do it all in one go.
That is what I would expect, a place that you need to go back to, that is tough as all get out to deal with, and each return trip would take you in further - probably via different planes. First trip via the "Middle World". Second via the Shadowfell, and the last via the Astral Sea.
Heck I can see some choices for each tier of play then...
As to making the Tomb tough its about the choices.
If you want it to be tackled by 15th level characters the main traps better be 20-21st level in rating. Not 15th.
Every encounter should be in there on the hard setting - nothing that isn't a minion should be 15th level, the XP budget should be 17th level and the basic creature/trap should be as well with "boss" encounters having things up to 21st level in them. Allowing that the adventure is going to then take about 8 encounters to level a party, if you aim at 8 encoutners per layer of the dungeon you then get to bump the challenges up each time you transition a layer.
Consider the following grouping just from the DMG:
Elite Altar of Zealotry
Field of Everflame
Kinetic Wave
Position the Wave to Push into the Everflame which should surround the Altar which is the key to passing through the room.
Then you have a Dominating/Dazing Trap, a Pushing Trap that Knocks Prone in Blast, and a Start of Turn/Move into Trap. If the whole party gets dominated and fails their save they are going to die. Remember that Dominated targets can be made to attack each other...
Not sure that qualifies as "fun" but its certainly brutal and likely to be lethal to 15th level characters, and is on the high end of the Hard Range for 15th level characters in terms of xp award.
I also wouldn't be shy about combining things like the Entropic Collapse with nasty Undead for a higher tier fight as well.
Consider this for 17th level PCs or as a real brutal encounter for 15th level ones...
1 Entropic Collapse (Level 23 Trap)
3 Bodak Reaver (Level 18 Soldier)
1 Lingerer Fell Incanter (Level 18 Elite Artillery)
You now have a trap that occupies 10 contiguous that is triggered only by PCs, and monsters who can move through the trap and use their powers at leisure. Particularly note that the Lingerer can set up the whole party for the Bodak's with some luck...
They are even logically the reanimated corpses of prior adventurers..
Insta kill - no, but tough fight that will likely kill a level 17 party - you bet.
In short it really isn't hard to threaten or kill PCs in 4E if you really want to, its just really hard to insta kill them, because the rules assume that for most players that isn't going to be fun.