A product may have multiple initiatives and then you realize that they are very similar, but different, and you want to create an intra-product hierarchy. Same product -- multiple, related initiatives.
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The roll-up feature was designed to be used to tie strategy from products to higher product lines. This allows you align your strategy at each level of the product hierarchy.
One workaround could be to create custom fields for your initiatives and tag related initiatives so that you can report on and visualize the relationship.
At this time, due to current priorities and community feedback, we are unlikely to make updates in this area to allow for initiative rollups within the same product. We hope you can understand.