Where would you publish a serious research proposal/idea that you don't have time to work on yourself, but would still like to get credit for?
Does a solid, well substantiated research proposal count as a legitimate contribution? 


No, it does not.  A plan for achieving something, where none of the steps have been completed, is not a significant achievement.

Sometimes it is necessary to do experiments or calculations in order to form a plan.  That could be an achievement.