If by "good idea" you mean a gimmicky or complex map, I don't recommend it, if you can grasp hammer but are not very experienced; making gimmick/custom gamemodes will be hard and too much to handle.
If you mean good idea as in any map; try copying the Viaduct formula as a slate and if you want to innovate, twist minor things like pickup positions, capture/spawn times or even the ways you walk into the Point(s)
Personally I went through this "phase" 4 years ago when I first got into mapping, I believed I not skilled enough to pull anything off so I was mostly playing with the map editor and screwing around with bots on a simple layout, I still have the VMF Files in my folders too.
My opinion is just do that, copy Viaduct formula if you really can't come up with anything, don't overwork/stress yourself and play more official maps and as different classes to get ideas or to spot cool pieces of the map with good gameplay (AKA Inspire yourself)
Oh yeah, also, I recommend you use Dev textures and not decorating your map until you have a good layout, because if you're not satisfied with the layout and you already decorated it it'll become frustrating to throw away a lot of the work just to make a tiny adjustment to the gameplay.
