I disagree jonnylipsham.
I don't think it would render the flagship version useless at all, on the contrary, I think it would make it more accessible. Effectively it is still buying the flagship version, only it can be done in installments, the final result would be that most people would eventually own the Pro version. For example, if the project page is considered 70% of the value of the pro version, then simply set the price of that add-on as 70% the cost of pro. However, some people might not require this feature in the short term but might want to try the video player for a project and if this feature is the second most valuable feature, it might be priced as 20% of the pro version etc. Later on they can go back and purchase the project page and once you have purchase all the add-ons you would have essentially 'unlocked' the pro-version.