(this is id1 with no mods) have a bunch of switches hidden in the level that modify the item in a secret. by moving the items about on funcs. The item in place in the secret is upgraded depending on the number of switches pressed, then when the player is finally ready to collect the item they press a final switch and commit to that item. When the final switch is pressed, the other switches are all killed (maybe the items can be killed directly, but I never tried) so the player can't get multiple items.

You could even do something evil like give the player a rotten health if they find a bad switch (or get them all as punishment for being too good.) I can imagine using this before as a regular level puzzle that is announced to the player at the start of the level and treated as a challenge. "PRESS THIS BUTTON ONLY WHEN YOU ARE READY!"

I'm not exactly sure how the thing that moves the items about would work though... Could just kill the old item and drop new items from above I guess. 

"using this before a boss encounter" 

I guess you could have the floor where the items appear be illusionary and have multiple lifts underneath that bring up items. When the next switch is triggered the current lift and items are killed and the next brought up. You could also have a door that comes down over the items to hide the switch, so that it looks like some kind of magic trick. That would be kind of cool I reckon. Might work nice in an Egyptian themed level or similar.

The items and switch to commit to those items could be in front of a door to the final encounter. If you press the switch when you start the level, you get no items and have to tackle the encounter with sg, 25 shells and your axe :) 

While complicated setups are certainly cool, you could also put the whole set of possible items in the secret area and killtarget all but one depending on the player's actions. Saves you the trouble of messing around with doors and illusionaries.

At least this is how the items are changed depending on game mode in the mappis. 

Yeah, but I was suggesting it would be cool if the player could see the item they would get if they press the switch to commit. They might see the foreboding door in front of them and think "hmm, this isn't enough". 

I doubt any player would understand what was going on ;) 

I'd love to see the new than Egypt map.