The Mendix runtime and the native app are tightly coupled. That means if you wish to build the native app in Mx9, you will need to create a second app besides your Mx8 app. From a development and maintenance perspective that route is probably a good idea, depending on how complex the native app will be. Because the native app synchronises the domain model to it's local database, you will need to duplicate data from the Mx8 app to the Mx9 app. I don't have extensive experience with datahub, but I don't think that is what it is intended for. Setting up Rest services between the two apps is definitely possible, and will give you the advantage of keeping the Mx9 database clean, since you can analyse which data you will need in the native app and set up your domain model without having to worry about conversions etc..
Is it the same web app which you want to build in native, with a common database?
If they are different then I recommend to build the native app in 9.x.x and then use webserivices to get data to and from the webapp. I am not a native expert but I think even though you have some similar features which you might want to reuse in the native app, it is still some efforts to build it in native.
Yes, we would be sharing a common database. But only certain features would be available in the Mobile App.
Should I publish Rest API from the Web App or is DataHub a good route to build Mendix Native Mobile Apps?