Another thing you have to consider is how interested anyone in future generations will be in carrying this multi-generational project forward. In my experience, most
family members just aren't that interested in the details of previous generation members' lives, thoughts, etc. However, a positive in this is that -- given our currently technology, we have the capability of more easily producing longer-lasting, more interactive, richer content. I've also noted that people's interest in this sort of thing generally increases with advancing years. My father was in his later years before he really got interested in our family-tree going back 8+ generations.
Towards addressing the concern of electronic data format obsolescence, you may be interested in the work of the Task Force on Digital Archiving.
The foregoing hyperlink connects to an interesting PDF report on their findings and suggestions. Of special interest to them and, I think to you (judging by what it sounds like you're attempting to achieve), are the concepts of Content, Fixity, Reference, Provenance and Context.
Really what you need is some widely-used standard data structure that you can count on being in use down the road a few decades -- and this is the weak spot. As Carol said, things are becoming proprietary. Everyone wants their piece of the pie, so to speak. If we had a guaranteed data structure standard, you could then confidently count on your heritage data being retrievable and writable, no matter what particular format succeeding generations decide to mark-it-up in (HTML, holographically, etc.) British librarian Adam Farquhar summed the problem up this way:
"Einstein's notebooks you can take down off the shelf and read them today. Roll forward 50 years and most of Stephen Hawking's notes will likely only be stored digitally and we might not be able to access them all."