I plan to join and will probably arrive from Romania.

I will combine it with Turkey (Istanbul, Bursa, Edirne) and Greece (Philippi, Meteora) - aiming for 10 new WHS!

Meteora is the missing world class site of your trip ;)

Yes indeed. Although I am also looking forward to Bursa.

My wife and I hope to be joining as well!

I most probably join. But September is far off, so I must confirm a bit later.

I was planning to visit Edirne on the same trip. But it seems to be a problem to cross the Bulgaria/Turkey border with a rental car. Does anyone have experience with that? Maybe a rental car company that allows border crossings to Turkey? Did someone plan this as well and came up with a better solution?

If you do a search with a car rental agency like Argus you will find quotes from a wide range of Bulgarian rental companies so you can then easily see the various "rules" applied by each of them (click on Ts + Cs). I think you will find that NONE of the "international" companies will allow transborder into Turkey. Even for some of the local companies, Turkey seems to be a no-no. I did find a couple however (search from Sofia Airport) -
Autojet - 65 euro extra
Top Rentacar - with BIG costs and restrictions - "Cross borders are allowed to Turkey, Greece, Romania, Macedonia, Serbia on request. Cross borders for the class MCAE are not allowed. There is an additional administrative and permission fees apply: 50 euro for economy vehicles (EDMR,HDMR, EWMR), 60 euro for compact, estates, intermediate vehicles (CFMR, IDMR, EDAR,EDAH) and 80 euro for all other vehicles (IDAR, FWAR, FWMR, FDAR, IVMR, FVMR, IFAR, FFAR). The mentioned fee is for 1 country only. For each additional country the fee is increased by 50%. The renter should provide us with a copy of his driving license and ID card by e-mail at least 3 business days prior arrival. The required security deposit is double."

We have done/investigated a lot of multi country car rentals in recent years (including most Balkan countries) and cross border restrictions or add-on costs seem to be becoming the norm. The name of the game is to make the headline cost as low as possible and charge extra for anything else. And cross border, especially to beyond the EU, adds a lot of risk - which is not insurable for a reasonable cost. Car rental companies from e.g Carcassonne wouldn't even let me take a car into Andorra recently without paying more!!!

If you only want to go to Edirne then look at driving to the border (Kapitan Andreevo) or nearby (Svilengrad), parking your car and taking a taxi to the border itself, walking across and then getting another taxi to Edirne - it is only c26kms. LP and other guides tell details - it appears that Dolmuses to the border from Edirne and vv are plentiful. You won't be the only person doing it and the Bulgarian "cost" of paying for a rentacar for a day to sit doing nothing is very little!. We were in Edirne itself 2 years ago - though my last experience on crossing this border was back in 1965!! You wouldn't need more than a "full" half a day in Edirne so you would have time to get to and from the Bulgarian border from elsewhere in Bulgaria, cross and back and see Edirne all in a day

Solivagant
Thank you very much! I think I'll go for the "park and take a taxi" option.

Great, I m currently in Corn 🌽 Island Nicaragua, but have booked the
flights now in September to attend the Varna meeting, going to Thessaloniki
from Basel with Easy Jet, return from Sofia with Wizz together
about 50 USD just hand luggage. I plan to visit Mt Athos on Sept 2 or 3
if someone wants to join, then maybe go to Turkey for 3 days
depending on what sites the group will visit in Bulgaria I might also just
visit the remaining sites or Tentative sites, cheers and 🍺

Great, boracay (Thomas) - I'll put you on the list

I just noticed this article in the WHS in the News section on the home page and thought I would flag it up as many people seem to be planning to visit Edirne as part of the meet up:

Selimiye Mosque to go through extensive restoration (09.02.18)

It will still be open, and I doubt it would put me off a visit, just thought it worth flagging up.

Thanks meltwaterfalls! I will try to find out how the mosque looks like in september. If there's too much scaffolding I might skip it. I did not enjoy my visit to Tournai at all and don't want to repeat this experience.

I will be coming to Varna and have made appropriate flight bookings; to Izmir to see Atila and Nihal and visit Aphrodisias, and home from Thessaloniki, having visited Philippi en route. I can only manage a paltry 2 new (to me) sites on this trip though I'll chalk up another 7 in Canada and Iceland in June. At present both Eges are planning to come to the meet-up as well.
I look forward to seeing friends, old and new in September.

Looking forward to see you too again, Iain.