From what I've read, Samus Returns met exceptions. Metroid has never been a big series with the first Prime game being the best selling, and 2 million worldwide. So their expectations for Metroid isn't going to be sky high.
I don't see them going to go 'all out' for a series that best game sold about 2 million. Even if this is the best selling Metroid, it can be that if the game sells 3 million. Very good, but not worth a giant marketing budget that can eat into profits. Not helping is that Metroid is extremely niche in Japan.
This generally summarizes my thoughts. Personally, I think Nintendo will probably be aiming for 3 million in sales for Metroid Prime 4. I'm more than certain they are aware of the pedigree and reputation of the Prime series, in addition to being mindful about expectations of HD titles. Xenoblade and Fire Emblem received decent marketing campaigns. I also feel Nintendo is building an audience that would be more receptive to Metroid than other Nintendo systems in the past. Titles such as BotW, Smash Bros, Splatoon, Wolfenstein, Doom, Bayonetta, Resident Evil titles, Metroidvania indies, metroid on NES online, and possibly MPT HD are titles that should help build an audience that would be more predisposed to buying MP4. While not that significant Metroid did get 2 new reps in Smash as well. In addition to the Nintendo fanbase being consolidated on one platform and releasing on a more succesful system than Gamecube at the very least.
In general, I think that Fire Emblem, Metroid Prime 4, Luigi's Mansion and Animal Crossing have the potential to reach new series highs. I think titles like Xenoblade, BotW, Octopath Traveller, Fire Emblem Warriors, Pokemon Let's Go, Tales of Vesperia, Skyrim, Diablo 3, Fire Emblem's presence in Smash and the Final Fantasy ports will help build a large audience that would buy Fire Emblem. I dont think a Nintendo platform has offered as much variety in terms of different rpg styles and audiences in the past. Not too mention Fire Emblem Heroes. Titles such as Stardew Valley, Harvest Moon, Splatoon, 1, 2 Switch, Pokemon, Mario Party, Smash Bros and Mario Kart have built a good audience for Animal Crossing. In addition to the last new Animal Crossing releasing in 2013. Since then Animal Crossing has been present in two smash games and two Mario Kart games. As well as Isabelle having her own spinoff that was pretty popular, an Animal Crossing party game that bombed and a mobile title. Finally, titles like Super Mario Odyssey, Mario Party, NSMBU DX, BotW, Mario Kart, Smash Bros., Resident Evil titles and Captain Toad are helping build a diverse audience that would be receptive to Luigi's Mansion 3.
In terms of potential expectations, I could see Fire Emblem and Metroid Prime become 3-5 million sellers. Both series current series highs are at over 2 million. Animal Crossing Switch could be a 15-20+ million seller, imo. The series on handhelds were selling between 10-12 million, console entries and spinoffs all sold a few million. The series has been on a huge upward swing and I think it could explode in the same way Smash has done. Luigi's Mansion could be a 8-10 million seller, imo. Dark Moon on 3DS sold over 5 million copies. I think Nintendo will make the proper investments into the title, given how much Dark Moon sold. Obviously all these expectations rely on a lot of unknown variables.