I'm not a software developer and I don't work in this business but I would be curious to know, according to the manpower advertized by FDEV, what could be feasible or not for this update considering the long wishlist of the community:
Without knowing the internal structure of the code base or how those 100 devs are distributed between programmers, designers, artists, etc. it's very hard to say. So, broadly, think about how much got done in Horizons (2 years) or Beyond (1 year), and expect something on that sort of scale. It'll be big - the entirety of Horizons delivered at once would have been huge! - but don't get too excited yet.
Some things to consider
[*]Atmospheric Worlds
[*]Space Legs: combat and ships interactions, social areas in starports.
[*]Base Building
A big and often overlooked challenge for these is the amount of new art assets required for them (or in the case of space legs, the number of existing assets which need a lot more detailing added so they don't look terrible when right in front of your Space Nose).
There's a lot of re-use of assets in the existing game, which suggests that art (modelling, textures, shaders, rendering, etc.) is a big limitation on how fast things can proceed.
Base Building feels like it could potentially reuse a lot of the existing stuff, so would be easier from that perspective - while Fleet Carriers will demonstrate that Frontier has a solution to some of the big conceptual issues around it. So that's probably the easiest of the "big three" in that sense.
[*]New Missions (for legs and atmospehric worlds)
I'd just count this as part of implementing those features at all.
[*]New Ships
[*]New SRV's
Quite possible, again, art time is likely the bottleneck - sticking the stats together is relatively quick. On "new ships" the question is where the gaps are in the existing ships that a new ship could do - or would it just be "a Clipper, but made by Lakon"?
[*]Universal Limpet Controller
Very much depends how it's implemented. Figuring out a way to control one within the existing interface, given the differences between limpets in terms of behaviour, could be quite tricky. Also no indication from Frontier that they
want to do this. But probably nothing major stopping it if they want to do it.
[*]New gameplay on planets (SRV's AI)
I expect the AI team, like the art team, is one of the most time-critical ones for anything, just because of how much AI needs to account for and interact with, and the difficulty in starting until a lot of that is already in place. I'd expect this to be restricted to things around the major feature(s) of the upgrade, and maybe some bug fixing.
Obviously if the major features are on planets, they'll get some AI.
[*]New exploration contents
With the art constraints, probably only associated with the major features. Though perhaps comets will make an appearance too?
[*]Black holes with accretion disks
Possibly. Getting the graphics and gameplay right for the ones where the accretion includes a nearby star might be tricky, though.
Tricky rendering and instancing consistency problems unless they're as fatal as neutron cones to end up in, though - I don't know how long it took them to get ring systems working.
[*]Thargoid motherships combat scenario
If the motherships are included at all, yes, most likely.
[*]Powerplay v.2
[*]BGS v.2
BGS changes of some sort, almost certainly. Powerplay changes along the lines of Sandro's proposal, possibly.
'v.2' could mean absolutely anything (and arguably we're already on at least v.3 of the BGS) with scopes ranging from trivial to impossible. So probably not.
[*]NPC Multicrew visible and operating in cockpit
Technically probably quite easy - the question would then be whether they took up a seat that a player multicrew could have had. If they get an extra seat added for them (easier with some cockpits than others) can that also be used in player multicrew?
Wholesale replacement is highly unlikely - but additions and improvements to the existing one are almost certainly going to be in there somewhere.