A "fully annex" button would be amazing. Especially when fighting colonial powers it is something I have craved for a long time. If you fully annex an overlord, all of their subjects automatically become yours. When taking out a major colonial power this can be amazing, but what I hate is when you select all of their provinces in Europe and in trade company regions that you know about, and the peace deal still doesn't switch to "we demand their full annexation". In such a case I have to scour the globe for some tiny insignificant island in the Pacific or elsewhere to fully annex them. Even more annoying is that provinces which they are colonizing also need to be selected. So even a province in Brazil which in the peace screen is indistinguishable from a province belonging to their colonial nation needs to be selected for a full annexation.
On a side note, it would be nice if you manage your colonies a bit more. What happens quite a bit is that Colonial Eastern America will start colonizing its way West, eventually entering the colonial Louisiana region. Often this then cuts off the expansion route for the Louisiana colony. An interaction which allows you to "enforce territorial jurisdiction", so basically force a colonial nation to surrender provinces outside of its own colonial region to the appropriate colonial nation (if one exists) would be very useful. It would also reduce bordergore. What could also be interesting is that much like how you have a native policy, colonial nations could also have a policy regarding treatment of the natives (this could be tied to their overlord's native policy, be seperate, or be something the overlord chooses via a subject interaction). So basically a colonial nation could focus on eliminating the natives which would cause the AI to focus on changing the culture of its provinces to the primary culture and convert as many provinces as possible or focus on coexisting with the natives in which case much of the native culture and religion would be preserved. Each policy could come with advantages and disadvantages. The conversion policy for instance could come with modifiers such as reduced missionary maintenance cost, reduced culture conversion cost, and increased missionary strength, but decreased tolerance of heathens/ heretics. A cooperation policy could turn off religious penalties, give more promoted cultures, and increase tax, but would result in a yearly loss in prestige and republican tradition and cause +1 national unrest.