Consider a special talisman item that can be placed within a level. If a lemming collects this item, similarly to collecting a pickup or activating a button, then this satisfies a talisman criteria.
The criteria itself may be defined as "Collect [N] talismans".
The object itself would have no impact on gameplay, other than encouraging players to go out of their way to find a solution which sees the item being collected.
This opens up possibilities for level creators to encourage "thinking outside of the box" when it comes to interesting (but totally optional) solutions.
From a certain viewpoint, this is already technically possible. Consider one or more cloner powerups placed at certain points around a level. If the normal save requirement is, for instance, "Save 50/50", then a talisman may require "Save 53/50". However, such a solution may cause unintended backroutes due to the power of the cloner skill. Further, if a level already has all its skill slots used up, then this is infeasible.
