If your players are familiar even in passing with the traps of the Tomb of Horrors, I suggest major rewrites of famous areas.
For example, four entrances rather than three, and an entry 'foyer' in all four so that the correct path is not obvious.
A third lethal foyer should not be hard to invent.
Similarly, I'd suggest that a new room be devoted to the devil's mouth in the area at the end of the first grand corridor, and that it contain not one but several mouths leading to various 'dooms' - though one or more of which might be a viable if dangerous entrance into the main tomb. Remember when designing things that Ascerak 'plays fair' to a certain extent in his fashion.
As for CR 15 traps:
1) Make the characters spend resources. For example, force them to clear corridors of magical darkness, magical obscuring mist, ect. If they don't, detecting traps is made much more difficult (-10 to -20 circumstance modifier on roll).
2) Use traps which are set off by magical detectors of various sort or which are triggered by mere observation rather than those that rely on pressure plates and gravity. For example, inscribe permenent symbols on the wall of a room containing magical darkness. (Ascerak would consider this playing fair only if all rooms containing magical darkness could be safely traversed if the darkness was still in place. 'Night's good color...' and all.) Or, a lightning bolt trap that goes off whenever a living creature enters a particular cube of space (this is particular effective in partially flooded rooms).
3) If you must use gravity traps, such as pit traps, they should be accompanied by anti-magic effects. For example, a corridor with pit traps or tilting floors might have an anti-magic effect over the whole area. More lethally, a corridor with pit traps might have magical greater dispelling traps that go off whenever an ongoing spell effect enters the cube of space above the pit. Another variation on this is the 'vacuum trap' which is a 'pit' which 'sucks'. So, when it goes off, not only do those standing on it fall, but those that are flying or standing nearby must make strength checks to avoid being sucked in. The bottom of the pit naturally contains some dangerous obstacle, such as a teleport trap, a whirling fan, grinding gears, or grill made of spikes and sharp blades that sucked/falling characters are thrown against. Note that one of the cool things about this sort of trap is that 'down' doens't have to be in the expected direction. So, a player could be using slippers of spider climbing in an attempt to avoid pit traps, only to find themselves sucked through the ceiling. In fact, IMO Ascerak's sense of 'fairness' is such that he might have a particular corridor which is safe to traverse only in the obvious straightforward way (although it may appear otherwise), and attempts to fly through it, walk on the walls or ceiling and so forth are all doomed to failure.
4) Force the players to fiddle with the trap. Getting through a closed door is the classic case, but players are likely to be so paranoid around doors (and in the ToH in particular), that any amount of traps are likely to be bypassed. The ToH is particularly well designed in this since it requires fiddling with various trapped objects in order to obtain the keys necessary to penetrate further, and for the most part this fiddling can't be easily bypassed (smart parties will figure out ways, but that's ok). Touch traps such as contact poison, electrified doors, and so forth are the best here but remember to play fair. There should be the tools to get around the obstacle somewhere in the room/dungeon or standard kit of a player. Don't design traps that you yourself don't know how to bypass. That's lame. Any DM worth his salt can design unavoidable traps. The trick is to design traps that work as puzzles.
5) When in doubt, the trap should have an area of effect. This is particularly true of group puzzle traps where failure is not individual irresponcibility and rashness but a lack of group wit. If its obviously trapped, then setting it off should effect a large area. Gas traps, magical bursts, crushing rooms, rooms in which the entire (conjured) floor of the room disappears (particularly simultaneously with a greater dispelling trap), flooding rooms (whether with water, burning oil, acid, or pyrotechnic flows), etc. should be the norm at high levels.
6) A careful group of high level characters is likely able to completely bypass a trap by going around it in some fashion. In ToH, excavation is a particularly obvious way. Ascerak should be prepared for this. For example, the inner tomb (from the adamant gates onward), might be embedded within a lava filled vault. Breaking a hole in the walls destroys the magical protections on the room (causing it to rapidly heat up) and allow the lava to begin to pour into the room. Upper chambers might have different but similarly lethal traps of this sort. For example, in the upper levels doorless voids within the hill might have been randomly scattered about and filled with swarms of hellwasps, green slime spreading cockroaches, undead spider swarms, wraiths, ect. This make total or targeted evcavation difficult. This fits Ascerak's 'style' by the way, since if the players don't play 'fair' then they should be punished.
7) To be honest, passive traps are very difficult to design which threaten high level characters, because they are well passive. They just sit there while the characters design ways around them, and high level characters just have too many resources for doing this. Conjured/summoned creatures as traps don't actually work, because there are too many ways around them (hense the CR on a conjured creature trap actually does top out somewhere around 10). Some ways I've found around this are to take normal creatures and then convert them into traps using polymorph/temporal stasis spells. These traps become active when thier magic is dispelled (either by the players fiddling randomly or by the players setting out a dispelling trap). I particular like traps which only go off if the player fiddles with them (for example, a trap that is only activated if the players damage the object that the creature has been polymorphed into). Another variation is the trap as construct/animated object. These sorts of traps can't merely be disarmed, sense they can rearm themselves. This can be particularly effective if the 'trap' has resources, like the ability to open/close the doors in the room, shake the room, and use the trap and/or spell like abilities. I don't think this later sort of trap is really Ascerak's style, but its something to think about particularly as a 'dead end' trap.
