What if Dragonair could learn Coil?
As someone who has used Dragonair extensively, I'd say that this would be the perfect boosting move for her. The main issue with using Dragonair is finding the right balance between utilising her excellent type, abilities and defences (with Eviolite) and giving her the right offences needed to cause sufficient damage after 1/2 DDs.
Splitting the two just leaves Dragonair nowhere near as potent. However, with Coil, Dragonair would become a firm bulky booster. Coil raising defence means that Dragonair can invest in SpDef as opposed to Speed, which along with the Dragon type allows her to be an excellent special wall, resisting Surfs, Tbolts and Fire Blasts.
Like you mentioned, Dragon Rush becomes the STAB move of choice, which is perfect for a bulky booster as opposed to Outrage, providing excellent power and no drawbacks, with great neutral coverage alone.
Finally, Dragonair has an excellent defensive ability in Shed Skin, which allows her to utilise RestTalk to cover the issue of recover that most bulky boosters face.
Currently, Dragonair lacks definitive direction in how to be utilised. Coil would give her a "standard" set that would very effective. It would function similarly to CM Musharna did a few metas ago, except with a much better type both offensively and defensively. And well all know how good CM Musharna was/is.