@B_Sp Lookup is just a field that populated based on relations, but we are not going to make it bi-directional, in this case it will just duplicate relations functionality and make things more complex.
However, what you need is a Lookup Collection (as we call it internally), this is something we are going to add soon. In this case in Objective you will be able to add Ideas Lookup that will collect all Ideas linked to Initiatives linked to this objective. Note that this will be not a real relation, but just a way to extract information from lower levels.