Rules for Nodes & Branches:
Here are a few rules for guidance. 

Engines by size. (1.25, .625, 2.5, 3.75, 5, 7.5, 10+)
1st node is basic engine, coupler, adapter small tanks
2nd is efficient engine, bigger, more gimbal, medium tanks & stack seperator  
3rd is largest engine, largest tanks

Probes by node:
1st is SAS only
2nd adds retro & prograde
3rd adds normals & radials
4th adds targeting and node

If the probe special promotes the part such as: built in solar panels, grabber, large SAS, RCS, etc...

Manned Flight
1st adds 1 kerbal orbiter w/parts
2nd adds 2 kerbal orbiter, very basic station parts
3rd Splits to:
 3 kerbal orbiter
 1 kerbal lander
 EVA tech
 basic station parts 

Again, special characteristics in a pod/lander can promote it.

Station branch progresses with recyclables, and extra life support, bigger and better parts leading towards permanent habitation. 

Antennas branch is by range:
1st is up to 15 Mmeters
2nd is up to 50 Mm
3rd is up to 100 Gm
4th is above 100 Gm

Renewable Resources (Solar Panels):
1st are fixed and up to 1 Ec/s
2nd are tracking and up to 15 Ec/s
3rd are up to 30 Ec/s
4th is above 30 Ec/s

Docking:
1st are small docking ports (0.625)
2nd is medium to large ports (2.5)
3rd node is for special ports (movable, shielded) and ports larger than 2.5 meters

SRBs - Calculate the potential of the SRB by multiplying the Thrust(ac) by the Solid Fuel:
1st node is up to 30K
2nd node is up to 100K
 Ullage - any small ullage SRBs
3rd node is up to 250K
4th node is up to 20M
5th node is anything larger

RCS & SAS - This is a dual branch unlocked by either rocket engines (RCS) or flight gyros (SAS)
with a common node for sevice modules containing a supply (generator, solar panels...) and either RCS & SAS.
RCS Nodes
1st are basic RCS thrusters
2nd are advanced RCS thrusters, verniers, 5 port...

SAS Nodes
1st are simple gyros
2nd is advanced gyros (10+ torque)

More to follow...
