A brief discussion on the forum mentioned the possibility of platinum and diamond talismans after I was shown a replay of my level "Deep Freeze" where all 80 lemmings are saved (anything above 78 requires extremely precise and fiddly gameplay).
Besides challenges that are harder than gold-level talismans (although this is open to debate), I am in support of a higher level, such as emerald or diamond, for those challenges that require ridiculous precision, but I'm wondering if actually a better option would be to have a level of talisman that is hidden in the player... that is, you can see the level has a diamond talisman, but you don't know its prerequisites, possibly because very few people know about the solution or the prequisites give too much of a clue as to what you have to do. For example:
No added colours or lemmings - complete without using Climbers.
Granted you can cheat by opening the level in a text editor or the NeoLemmix Editor, and there may be other ways to disguise the solution. Still, maybe something can be salvaged from this.