A minor feature idea:
In a similar vein to that you plan to have pre-placed lemmings, one-way arrows, secret level triggers, hint arrows and pickup skills as separate features instead of being part of the graphic set, is it also possible to make "markers" that you can place by pre-assigned trapdoors? These markers are no-effect objects.
Currently, the only way to mark that (e.g., as shown in the picture) gliders come out of a trapdoor is to put the gliders pickup skill on top, then put the object as Fake to prevent the lemming from picking the skill up. This can be helpful, but at the same time, since it is a pickup skill, it can be confusing (as there is also a stoner pickup skill on the right). The level in that picture have only has the pre-assigned gliders, but there could be cases where levels have trapdoors that have lemmings assigned different varieties of permanent skills (e.g. one trapdoor has gliders-climbers, one trapdoor has swimmers-disarmers-floaters)