If I use the GPSPoint code, I can concatenate gpspoint files and make them into layers just with a text editor.
Also, any new features I just add to that.
However, be mindful that GPSPoint looks for EOF.
I could modify the code to stop on a line that begins with "-"

Style 1: Use GPSPoint code

type="layer" id="aggregate" name="My \"Name\""
type="layer" id="trackwaypoint" name="cool"
type="waypoint" ...
...
type="layerend"
type="layerend"

Style 2: GPSPoint++

+Layer Aggregate
=This Is A "Name" with "Quotes" and WEIRD *(#$% chars

+Layer TrackWaypoint
=My CoolLayer

type="waypoint" ..
type="track"

-EndLayer

-EndLayer

comments (#) ignored
each layer parser must return control if line begins with '-'
However, this is to freestyle.

Style 3: GPSPoint to the next Level

+Layer Aggregate
=This Is A "Name"

+Layer Coord
=My Cool Chord Liar

type="param" name="deg_inc" value="5"

-EndLayer

-EndLayer

I'm a bit foggy on the parsing, does each layer do that? 'cause TRWLayer will have to.
also, if we do that, we may as well use #1 with \\'d quotes as it is more standardized.

Style #4: Happy-Medium

+Layer Aggregate
name=This Is A "Name" with funny =-10!@$#* chars

+Layer TRW
name=My Cool Layer

draw_tracks=1
draw_waypoints=0
data_format=gpspoint

+LayerData # parsed by Layer itself

type="waypoint" ...

-LayerData

-Layer

+Layer Coord
name=My Cool Chord Liar

deg_inc=5

-Layer

-Layer

Every thing is done by the file parser, EXCEPT LayerData blocks.
Everything else is done by some sort of settable resources.
Layertypes would have to list resources and types.
Unknown resource are simply skipped.
Unknown Layers are simply skipped until the next '-Layer', include their LayerData blocks if they have one.

comments?

Issues:

#1
One start character would be nice, like
!startlayer
!endlayer

etc.

all i can think of for now.

Style 5: Win Conf Style

[Layer Type=Aggregate]
name=Hello World

[Layer Type=TRW]
name=My Cool Layer
draw_tracks=1
draw_waypoints=0
data_format=gpspoint
[LayerData]
type=waypoint ...
[EndLayerData]
[EndLayer]

[Layer Type=Coord]
name=Chord Liar
deg_inc=2
[EndLayer]

[EndLayer]