I'm not sure I think this ought to be a full on EP, but a GP definitely. I would love a midlife stage, where sims grow a little grey, have a little thinner hair perhaps (more hairstyles for middle aged and elder sims), a few more wrinkles and cannot have kids any more (no more babies born the day someone ages up to an elder, please!)
For middle age I'd love various mid-life crises options, similar to phases but tied to their personality. So non-commitment or romantic sims could get the whims to have affairs, flirt, get divorced, get a young lover. Materialistic sims get whims to buy cool stuff. Some sims would get whims to quit their job and go back to uni, others to retire early, some might want to see the world, others dress like a teenager... (there could be a lot of cool options here).
For elders I really want a reward trait based on the life led - so they could be nostalgic for old times, bitter about chances lost, want to catch up on everything they didn't do, family orientated sims could get the trait "grandparent" that would open up new interactions. I would also love more grandkid-grandparent interactions overall.
