Hi,
nice to have some kind of rendered base layer without streets.
Yes, and we could have e.g. 2 layers only with streets:
1) "normal" street rendering like currently in mapnik 2) "physical" street rendering, based mainly on surface tags (this could be identical with the "bicycle map", if used with another transparent layer with bike routes)
We did not yet discuss which elements should always be rendered, but I think for a start that would be landuse=, natural=, building=...
That's what I am currently rendering in another project as well. "Just add water" -erm- I mean: just add waterway=
I would refrain from rendering additional info in a base layer, e.g. omit things like shops, housenumbers, post_boxes as these can be overlays.
In addition, I propose to supply for a coloured as well as a black- and-white base layer. I found it is very handy if you present various non-topological information, like coloured parking lanes, and the colour information stays out from the map.
Thomas you can also try to use b/w tiles http://toolserver.org/tiles/parking-bw/17/68646/45898.png as a base layer for your disabled map and see how it looks. (But as I look, I see that the parking map has too many things in it to really deserve the name "base map", and some coloured too)
Greetings from Würzburg, Kay