Thus you mean that anybody should use OSM for political purposes by suggesting new cycleways here and there? This is very much against the common principle of only mapping what's "on the ground" (see note about proposed features below). If you want new cycleways in your area contact your local politicians or otherways make local campaigns for such projects. That is much more likely to succeed than drawing fictious lines on the OSM map. Others may even think that there is really an approved project when they see such a dotted line on the map.
There is already a very established tagging scheme for proposed highways including cycleways: highway=proposed in combination with proposed=cycleway. As far as I remember this is even rendered by Mapnik. However, please note that this tagging is intended only for highways that are really expected to be built in the near future (i.e. the project has been approved by the local council or by government but construction work has not yet started). --polderrunner 08:15, 10 April 2012 (BST)