I'd be in favor of an accurate calendar, whatever it's name is. Where days have 24 hours and weeks have seven days and an atomic clock measures out the years prudently so the seasons don't drift. Julius Caesar fixed an inaccurate calendar 2100 years ago. Gregory improved it 400 years ago. The creation of leap-seconds fixed it further. I get it that Protestants didn't like a calendar with a pope's name on it. So they resisted and delayed. And some Orthodox like a pope's name even less. But I do think celebrating Easter on the same day is a powerful witness. The biggest witness of that would be that to adopt it we would have to reduce our dislike of each other enough to agree. Wouldn't be all Jn 17, but a baby step. I see what you say about different calendars reducing crowding. That's practical. But I think the value of an agreeable date, especially after disgreeing for so long, is bigger. Yet I really don't think it will happen. It could. I just won't see it.
There are legitimate objections to the Gregorian Calendar from the Orthodox, and also I would note the Ukrainian Greek Catholics and Russian Greek Catholics and some Ruthenian Greek Catholics use the Julian calendar, and others, such as the Romanian and Bulgarian Greek Catholics, use the Revised Julian Calendar, and this has the effect of aligning the feasts in those countries with the Orthodox Church.
Also in Greece and Syria, apparently all Catholic churches, even those belonging to the Roman Rite, use the Julian Calendar, so that the public holidays in Greece line up, and in Syria perhaps for that reason or perhaps for safety in numbers, since both the Syriac Orthodox and Antiochian Orthodox churches are headquartered in Damascus, and after the genocide in Turkey most Syriac Orthodox are now concentrated in Syria and Iraq, with small numbers remaining in the area of Tur Abdin in Turkey where some of the monasteries are still (barely) operational, and also a healthy population exists as one of the ethnic minorities in Lebanon, where Antiochian Orthodox are one of the larger denominations, with Maronite Catholics being the largest, and in Israel, where all Eastern churches are on the Julian calendar except for the Assyrians and Eastern Catholics, there is a minority population of Syriac Orthodox concentrated Jerusalem and Bethlehem, predominantly with Palestinian passports.
The main reason not to change the calendars is the last time this was attempted, it caused the Old Calendarist schism, which has been a nightmare for the Orthodox which has personally affected me, and the idea floated a few years ago to fix Pascha to the second Sunday in April could cause an epic multi-denominational schism since unlike the Gregorian and Revised Julian Calendars, it would be impossible to argue such a change was compatible with the acts of the Council of Nicaea.
I strongly favor maintaining the status quo in terms of calendars to avoid schisms, but as I said earlier, there are other benefits, like reducing overcrowding at places of pilgrimage. Also, it is beneficial if not all Christians are off work simultaneously on Pascha, as I believe it is of benefit in our modern technological society to have Christians in safety-critical supervisory roles at all times, as many as possible.
I believe that communion can be restored between the Orthodox and the Roman Church, and the key is baby steps, because while restoring communion is great, causing a schism is not so great. Most likely, the Assyrian Church of the East, the Syriac Orthodox Church, the Antiochian Orthodox Church, and perhaps also one more autocephalous EO church, maybe Alexandria, or the Czech Lands and Slovakia, or Albania, all of which would benefit from communion with, but not subordination to, the Roman church. For reconciliation to happen it will require a recognition of autocephaly and it will also require a conservative Pope along the lines of Pope Benedict XVI or Pope John Paul II. I am hoping that the successor to Pope Francis is the Patriarch or Metropolitan or Archbishop of one of the Eastern Catholic Churches - the Ukrainian Greek Catholic Church in light of what has happened, or the primate of the Ruthenian Greek Catholic Church in Europe given that it is spread the region, owing to neutrality and peacemaking potential, since the Ruthenians, also known as Carpatho-Rusyns or as Carpathian Russians, or as Greek Russians, who include several subethnicities such as the Lemkos, had been predominantly Catholic since the Union of Brest, but in the US a great many joined what was at the time the Russian Orthodox Church and later the Greek Orthodox Church, and they are spread throughout the high concentration of Orthodox churched in places such as Wilkes-Barre, Pennsylvania, in parishes belonging to the OCA, ACROD (the American Carpatho-Rusyn Orthodox Diocese, which is a diocese of the Greek Orthodox Archdiocese and unfortunately tends to have Greek rather than Carpatho-Rusyn bishops), ROCOR, and the small number of Patriarchal Parishes (there were originally 75 that stayed with the MP in the 1920s when the three way split among the Russian churches between ROCOR, the Metropolia, which later became the autocephalous Orthodox Church in America, and the MP occurred, but over the years that number has shrunk to just under thirty, under a bishop in the US.
At any rate, I believe a Greek Catholic Pope could push for the liturgical reforms urgently needed in the Roman, Ambrosian and Maronite churches, by restoring substantial parts of the Tridentine, Dominican and other traditional Roman masses, the traditional Ambrosian Rite mass used in Milan, which I think would actually be, if translated into English and modified to follow the Roman liturgical calendar, the ideal next-generation mass, because its Liturgy of the Word has three scripture lessons, an Old Testament, an Epistle and a Gospel in addition to various proper Psalms, and this would allow retiring the awkward three year lectionary, and the traditional Maronite liturgy. Additionally such a Pope would understand the complex and fragmented nature of the Orthodox churches since, as might be obvious, the Byzantine Rite Catholics, who often identify as Orthodox in communion with Rome (for instance, the Lemko painter Andy Warhol, who felt compelled to sit in the back of the Roman Catholic parish he attended lest someone call him out for crossing himself right to left according to the Greek Catholic and Eastern Orthodox liturgical tradition in which he was raised - fortunately there is more tolerance and the only church where someone commented on my crossing myself was an LCMS parish I visited, which amused me considering some Lutherans do make the sign of the cross including Martin Luther, although the person was not mean or condescending, they were merely perplexed as to why I was doing it). Such a Greek Catholic Pope would be in a prime position to negotiate with the Orthodox because of the understanding they would have of how the Eastern Churches function, and the stress they would place on liturgics.
By the way, there is an interesting and unusual film from the mid 1960s starring Anthony Quinn as a Russian or Ukrainian Greek Catholic bishop who is unexpectedly elected Pope after being released from a Gulag by the Soviet premiere, played by Sir Laurence Olivier. The film concludes with him stopping China from starting a nuclear war due to a food shortage by putting up the real estate of the Catholic Church as collateral in order to facilitate China being able to purchase food on credit.
The Shoes of the Fisherman, directed by the same British Catholic director who in 1976 released
Logan’s Run with Michael York (who I believe was also Christian) and Peter Ustinov. It was clear he was a devout Catholic, and the film was not offensive in the manner of recent productions about exorcists or the deservedly controversial
The Young Pope series with Jude Law and its sequel
The New Pope, which also starred John Malkovich - I can’t believe the Vatican allowed such risque productions to be filmed in its churches and gardens, but they did, for some indecipherable reason.
