General consensus (as Hasbro never admitted it outright) is that the third part of the WFC Trilogy was originally going to be all TFTM themed, with the Studio Series 86 figures that were released in 2021 originally meant to be in the line that ultimately became Kingdom, with the none of the Beast Wars figures originally meant to be part of the line (and with the Fossilizers likely originally going to be something more TFTM-appropriate than dinosaur skeletons, and named something else of course), the BW figures only added to celebrate their 25th anniversary after Hasbro previously failed to celebrate BW's 20th anniversary and likely didn't want more flack for missing the anniversary as second time in the row.For all its flaws in execution, WFC was presented from the beginning as a trilogy, with a clear mission statement and even a basic plot arc. IDK if that got shaken up a bit by the pandemic, as I don't recall whether the BW pivot was always planned as such a core part of part 3. The last two years are mush in my mush brain. Regardless, from the get-go, there was "[x aesthetic] plus Micros plus Weaponizer style components", which remained consistent through the three lines (albeit with Kingdom shifting to Core class).
The premise for Legacy as described by Hasbro in their livestreams is that dimensional portals have opened up and are bringing all of these non-G1 characters from other universes into a G1 universe to all team up together in a big multiversal war. It's kind of like the 2003 Transformers: Universe line (and sorta like RobotMasters too, with the portals) in that sense, but with all new molds instead of redecos.Legacy's very premise lends itself to a chaotic, reshuffling-prone feel, too, as it basically doubles down on Kingdom's "let's have a mix of lines/character groups" which makes it a bit tough to get a unified sense of what's in each wave or line. Not necessarily a bad thing, mind: it's been really cool to see Bulkhead, Override, Jhiaxus, etc revisited and in some cases revisited really well. And it's been refreshing to break away a bit from the "cross off the G1 checklist" game (although that's been good for collecting too). But rather than move forward one era at a time (say, a Kingdom-style "G1 and UT" trilogy), it's Everything Everywhere All At Once.
It's also like the 2008-2009 Universe line and the 2014 Thrilling 30 line that likewise made new-mold toys of characters from across the various different series to celebrate the Transformers brand's 25th and 30th anniversaries, respectively. Legacy's main difference from Universe 2008/09 and Thrilling 30 is that it's not celebrating the brand's anniversary, it's simply doing what those lines did just because (and probably because enough fans complained about the lack of non-G1 updates since Combiner Wars).