As I mentioned before, I have used CBsubs by Joomlapolis. It has options to sell plans as merchandize products. Those might be separate "plans" or even sub-plans in plans tree structure. This allows to sell products or additional options to a parent plan.
How to do that in Emerald?
Cross-plans were my first thought... but it seems to have different parameters and purposes, right? Price Affect does only reduce prices! Products would also require to add a value to the base price. Cross-Plans are not displayed in a hierarchy. I mean, its not visually exposed that it is a sub-plan or that it has a required parent plan. There are only options to "hide" plans or show a message hint "To get this plan you have to have subscription XYZ".
Suggestions & feature requests
Would like to see the CBsubs "product" and "hierarchy" feature in Emerald. Maybe the groups or a parent/child relation can solve this...!? At least more cross-plan pricing options are needed. i.e. add prices on parent plan price.