Edit: This was a fun write-up but recently I saw a leaked Thanos minifig head, so no Marvel constration.
I've been looking at the set lists for a few days, and I'd love for your theory about the codename LEAF theme to be constraction. When we get more information we can revisit this.
However there is something I noticed elsewhere in the set list that I'd like to mention here as it relates to potential 2020 constraction. Under Marvel Super heroes there are two 2020 sets (76140 and 76141) called Iron Man Mech and Thanos Mech. What interests me is the number of parts. They have 148 and 152 pieces respectively. Those are small mechs if they use system pieces.
The closest example I can think of is this year's 70665 The Samurai Mech from Ninjago's Legacy collection. 70665 uses a minifig as a pilot, which would work fine for Iron Man, but Thanos has been up to this point represented as a big fig. Unless the set includes a minifig version of Thanos (which is actually plausible, as LEGO made a minifig Hulk some years ago), I can't see a system mech with that few pieces able to accommodate a big fig pilot.
This makes me wonder if the label "mech" applied to these sets is really denoting that they are buildable figures like the recent Star Wars ones and Super Heroes ones in 2012. We have seen constraction sets with about 150 parts in the past, like 75111 Darth Vader with 160, 75525 Baze Malbus with 148, and 75533 Boba Fett with 144. These three had many technic and system components in addition to CCBS parts that elevated their part count. We could see constraction sets of a similar type with these two 2020 Marvel sets.
I think it is just as likely if not more so that these sets will turn out to be system and not constraction, but I wanted to put this idea here to see if anyone else thinks it is plausible.