So I've heard that at this point in time, there are only 20 known weapons left to release. With Nintendo's 1 weapon a week policy that equates to 20 more weeks of content updates, or in other words 5 months, which takes us to the 30th of June, precisely one year of content updates as promised. So I have a feeling that there may be some updates we don't know of or we'll get one final map release in July, giving us 5 final maps in the game (I'd like to think it'd be either 4 maps or 6, keep an even number). If so my money is on 4 maps, 2 final maps from Splatoon 1 and 2 new gimmick maps (much like how the final 4 in Splatoon all had some quirk to them). That'd bring us up to 20 maps, which is a round number, more than 1 (like they promised) and means we'll have 12 new maps and 8 returning maps, making a 60/40 split total.
If this is what happens I'd say it's safe to say we won't be seeing Urchin, Bluefin, Triggerfish and Saltspray return, as I'm of the belief that we'll see 2 of Splatoon's gimmick maps and 2 new ones. My money would go on Piranha Pit and Mahi Mahi being the final returning maps, with 2 new maps of other gimmicks, whether they have miniature cannons, moving platforms or a completely new and unseen gimmick introduced (helped by all the experimental Shiftys). Museum won't return because of Shellendorf, no way will we have 2 similar themed maps here, and Ancho-V was infamous in being laggy and full of disconnects so that's a no go either. I hope they will be extending the content updates if only because of the online switch over, and to give out all the content in the game and not continue to support it when demanding money will be a slap in the face all things considered. I'll keep my fingers crossed for Triggerfish but I'm concerned it won't be returning.