It is not possible to provide seamless integration, it is not just the issue of Geometry Kernel but also about how the features are created thorugh various operators esp Vsweep in Ideas will not have equivalent operator in Catia and likewise. What will you do with such features. If the company decides to go in Seamless integration, it means all the CAD softwares should provide same functionality.
What about tolerance? A body which is having free edges in Ideas may be shown as closed body in UG because of different Tolerance they are working at.
I say that it is a waste of time to look and aim to provide seamless integration. What is the use of such a technology. How many users and what cost???
May be some company can provide a level of seamless integrations between one or two CAD softwares and that too not 100%. They can't