Make them fully customizable and give Tess's stock to Ikora. Let her [Ikora] also sell mods to the ghost shells that can be either be purchased directly or as rare drops from doing any activity. Exotic perks like Guiding Light would only be purchasable and would require a bit of a grind, or could instead be a weekly bounty once per account. Sparrows should also be treated similarly with Amanda, along with new mods such as increasing sparrow speed, reinforced chasis, sparrow trails/horns, stealth drive (reduces enemy aggro range and/or their aimbot), etc.