Here are what I see of your requirements:
1. certain existing entities in drawing could be used as base information to create new entities;
2. When user select these entities, you want to show the possible new entities temporarily, as visual hint to help user to decide to choose one existing entity, from which a new entity would eventually be drawn permanently
3. User then choose one existing entity from all previously selected entities (with associated temporary new entity visually shown).
4. Once user makes the choice, a new entity is added to the drawing permanently and all other temporary new entities are gone.
There are at least 2 different way to do it, as I can see:
1. Use Transient Graphics in conjunction with handling Editor.SelectionAdded/Removed events. With this approach, you can draw temporary entities during the call of Editor.GetSelection().
2. Use DrawableOverrule. With this approach, you can enable your custom DrawOverrule during the call of Editor.GetSelection, so that the selected entities would be drawn with extra shape.
While either approach would not be very difficult to code, it is somewhat advanced AutoCAD .NET API topic, which I am not sure whether you are up to. I might be able to put some code together later (late today or early tomorrow) and post back, using one of aforementioned approaches (not sure if I have enough time for both).
