I'd tend see this problem from the other direction. Audax filled a goal-setting shaped hole, and with those goals achieved, it takes on an everyday character.
It is possible to extend interest through 'add ons'. Mastering GPS, or through blogging, but at some point the novelty dissipates.
There's a substantial skillset underpinning riding the likes of LEL and PBP, and a lot of satisfaction can come from mentoring newcomers. LEL showed that there is a groundswell of interest, so there'll be a lot of new recruits trying to get to PBP.
One problem of hanging out on this forum is that it diminishes the 'wow factor'. Mixing with a wider range of cycling enthusiasts might bring some of that back. Once you've done LEL and PBP a few times, family and friends take these feats for granted.
I need to refresh my own Mojo. At present I'm not looking much beyond a cataract operation on Friday. It's my second such procedure within three years, and it's made me feel older than my 59 years. I feel I should do something that impresses myself. I wonder if PBP might fit the bill, but it would mean doing a long ride this year, and I don't know if I can or want to do that.
In the meantime I've booked onto Semaine Federale. which is PBP without the pain. If you've never been, I'd suggest that. The first time is packed full of novelty. I've been trying to make sense of the booking arrangements, and even that's provided some stimulation.