From a marketing perspective I'm doubtful they'd do a Wintertraum 19 opening. The event simply being what it is, is a big enough pull for loads of guests. Marketing the 2020 summer season from the very beginning with F.L.Y. as the big new draw for guests would be more effective.
2018 saw them introduce a few more different offers for tickets and such to draw people in during a quiet year; which seemingly was rather successful. If there's nothing new for 2019 I imagine they may simply repeat that. Or perhaps the park do have something small planned?
Otherwise this gap between Klugheim and Rookburgh will be the longest period of time the park has ever went without adding a new attraction. But then considering the gap is inbetween their two largest investments by far its understandable. Also it could very possibly be that Rookburgh was hoped/intended for summer 2019 but as rumoured at the time they suffered serve delays with the groundwork in 2017 (similar to those delays Liseberg suffered with Valkyria's but on a such larger scale) - in which case it was just unlucky.
Also, in 2020 it are mostly the big parks which are opening something new.
Efteling is opening dueling powered coasters in 2020. Europa-Park is opening the new Piraten in Batavia in 2020. The first step in the masterplan for the redevelopment and expansion of Walt Disney Studios Park in Disneyland Paris: the transformation of Rock 'n Rollercoaster and it's area into Marvel.
In my point of view, going into this competitive year is equal to ending up in the end of the tournament. Daily visitors visit one or two themeparks a year so they have to choose. When they choose for the bigger ones like Europa-Park, Efteling or Disneyland Paris, they forgot about Rookburgh.
Maybe it has no effect on the number of visitors for Phantasialand, if they open Rookburgh in 2020 as well but I guess it will. Not much but still.
I wouldn't see a 2020 opening as bad for them. I don't see Disney Paris being serious competition to PHL, especially just the distance.
While their actual competition; the parks in the local German/Benlux area have nothing major planned. I highly doubt the appeal of F.L.Y. and the target audience of families with older children (who knows what the height restriction will be), teens and adults. would be deterred by Efteling's small kids coaster or EP reopening a dark ride. Obviously we don't know yet whether Toverland or Movie Park Germany are doing anything significant then but I doubt it.
2021 is the year to avoid at all costs - that's when Efteling have their major new coaster/area, Walibi Belgium's Intamin Mega and possibly Europa Park's new coaster and themed area open.
As for why they announced Hotel Charles Lindbergh when they did: I'd say Hixee has hit the nail on the head there. Firstly, other than the unveiling of the Berlin street gateway, it has been a year since we received any Rookburgh news officially. Give the fans something more to talk about.
Secondly, as is the case with most of their announcements, we will soon be seeing signs of the Hotel construction and the secret would be out anyway. This way they have control/own the news. Once the steel structure for holding the Tube Rooms, and the rooms themselves, start to appear on site we would have quickly figured it out.
Raik was only announced a couple of weeks before the supports arrived on site and erection started.
F.L.Y. was only announced a month before the erection of the first supports and track.
Also I wouldn't think the announcement it has any bearing on the regulations and time scale for when it opens.
For comparison Europa Park's new Hotel Kronasar opens 31st May 2019 and started taking bookings back in August 2018. Yet was announced years before, although the name and theme announcement was only in September 2017.
Or to time travel over 10 years back to when Hotel Matamba was built… Shortly after the start of construction of the hotel was officially announced 12th June 2007.
Matamba opened 23rd August 2008. The opening date was announced and bookings started to be taken on 19th June 2008.
Will be interesting though to see when they start taking bookings. Given the nature of the build and the emphasis placed in the press releases on Rookburgh being a "gesamtprojekt" it will undoubtedly all open at the same time - so I imagine it won't massively far in advance like EP until they know for certain everything will be done and operational. However given they will be wanting people to book hotel stays I can't see the opening announcement being as shortly before/only 3 weeks in advance as with Klugheim/Taron. Maybe by the end of Wintertraum 19/20 we might get confirmation of a start of the 2020 season opening? Or if it does by some miracle open before then, a couple of months beforehand later this year.