Rachel raised the question of keeping one's interest after the first heir moves out.
One thing about the decadynasty is that there can be long stretches of time when not much happens except the heir skilling or meeting job requirements or waiting for black ops or just waiting to age up to adult. I usually marry the spare to someone, move in the spouse, and have a child or two. The only person you have to be careful with is the previous heir, to make sure he doesn't max a skill that is someone else's supermax. Family members can work on the collection and make friends, but you can also have fun with them, sending them to a SimFest, or playing pool. When we get Seasons, there will be more things to do for Sims who don't have to worry about dynasty requirements.
Re-decorating the new house is another possibility. One thing I like about the decadynasty is getting to live in different houses. I usually just get them better beds and appliances, but if you enjoy re-decorating, you can have a whole new house to work with.