Your very rough math is very roughly correct. 1/3 developed, 1/3 left to develop, 1/3 never-to-be-developed.
That said, I think the capacity for expansion specifically as it relates to the theme parks is much more limited, not by space, but by time. I can't see them ever building beyond six days worth of attractions in the "core four" theme parks (i.e. excluding water parks and the like). That means a two-day MK, a two-day Epcot, a full-day DHS, and a full-day DAK. Anything beyond that doesn't make much sense because you're not going to attract a sufficiently significant number of new guests to justify the investment.
Well right now, it's (in terms of attractions)
MK - 1.25 days (1 9am-midnight day, 1 9am-1pm day... if you're not doing every single M&G and you know how to use FP+)
Epcot - 0.75 days (9am-5pm... should be done with every attraction if you do TT single rider, and FP+ for Soarin', MS, and SSE, and pick one up for Nemo or LwtL. I don't count alcohol guzzling over at World Showcase as enough for a full day
)
AK - 0.75 days (if you do EVERYTHING and gets FP+ for 3-4 attractions)
DHS - 0.5 days (FP+ for TSMM, ToT, Star Tours... single rider for RnRC, pick up a FP+ for GMR... see a couple shows... out of here now that OMD, Backlot, soon-to-be LMA/Mermaid, American Idol, Sounds, Captain Jack and Animation - all of the 'fluff' is either closed or soon will be closed)
The DHS overhaul should make the park a full-day affair - just will need some enhancements here and there.
The later operating hours (until 9pm or 10pm) will keep AK at 0.75 days - they'll still need to add a couple family dark rides, overhaul Dinoland (demo Dinorama), fix up Kali/Everest and possibly add a new land long down the road.
New Frontierland, a Tomorrowland overhaul/expansion, an Adventureland expansion and a B&TB dark ride (3 of the 4 are pure wishes... nothing more) would make MK 2 full 9am-12am days, even with FP+. But you could still do all of the headliners in 1-1.5 days - I say 2 because of things like Tiki Room, HoP, CoP, Country Bears, Laugh Floor, Philharmagic, etc.
Epcot unfortunately will only be 1 full 9am-9pm day of attractions if you bother to wait 3-4 hrs for Frozenstrom
So by 2021/2022
MK - 2 days
DHS - 1 day
AK - 0.75 days (once new 9pm or 10pm closing time starts)
Epcot - 0.75 days (unless you do Frozenstrom, then 1.25 days)
Only two parks will be full-fledged full-day parks... especially if the two Pandora attractions have 2,000+/hr capacity. I don't count 'eating/drinking around the world' as enough for a full day, way too expensive and ridiculous when you've already paid $100+ to get into the park... I'm only going by attractions (what most people go to the parks to do) with Epcot,
TDO is making progress, but there'll still be a long way to go unless Epcot gets a WoL revamp, and updates for Energy/Imagination in the near future. AK will be fine... it'll just need 5-7 more attractions slowly added over 15-20 yrs once Pandora is finished. Some big, some small.
