This is certainly getting hairy...I'm only travelling down this road until my tank is on empty...
Couldn't the Y-axis function as the W-axis? This would disregard the possibility of changing the EoT timeline, which I think is fine as all gates connect to a “present” EoT location and travel forward with it.
So travelling from 10AD to the EoT (assuming that 10AD is the true “present”), would yield (X,Y,Z):
(10,0,0) to (5,10,0). Spend 1 unit of time there and return to the normal timeline...(5.5,6,1) to (11,0,1) and then spend 1 more unit of time in the normal timeline, travelling back again to the EoT...(12,0,1) to (6,12,1)...something like that? (no back tracking on the Y-axis)
Gate to Gate or Epoch use on the normal timeline wouldn’t complicate anything as that is self contained...Gate/Epoch to EoT also wouldn’t complicate anything as they always arrive at the “present” EoT (assumption for this example would mean (1150,2300,0), since X is half of Y and the party can access 2300AD from EoT). So even if the adventure started in 12kBC (-12k,0,0) and they travel to the EoT, the coordinates for the present EoT are at (1150,2300,0). This limits the possibility to see far into the future, at least from the EoT. A gate to gate transfer into the far future would be ok me thinks.
Starting point (X,Y,Z) Time from EoT (assumes that 23AD is the “present”) perspective
10 AD (10,0,0) EoT (11.5,23,0)
Travel to 6 AD
6AD (6,0,0) EoT (11.5,23,0)
Spend 1 year in 6AD
7AD (7,0,1) EoT (12,24,1)
Return to “present”
11AD (10,0,1) EoT (12,24,1)
Travel to 23AD
23AD (23,0,1) EoT (12,24,1)
Spend 2 years in 23AD
25AD (25,0,3) EoT (13,26,3)
Return to “present”
13AD (10,0,3) EoT (13,26,3)