In Theory, you can achieve any game as a solo developer, even an MMO, if you spend enougth time and effort into it. The technical requirements are low related to the work which is needed to get decent Level Design, 2D/3D Art, Mission Scripting and Audio. Don't get tme wrong, you spend most of the time into those mentioned but the programming work is the most important and challanging work you have to do. There is no game without code and there is no good game without knowing not just the coding language but also some basic principles and techniques, sometimes also platform specific stuff, in order to get your game running well.
Don't underestimate any of the professions in game development!
The scope of your game and so the time you need to finish it is not just related to which kind of game you make but also what tools do you use. Unity is straight generic oritneted, so thai aim to be “the solution” for everything and for this reason, there isn't much implemented in a ready to use fassion for your specific kind of game. Means, you have to spend a lot of time to create the gameplay systems you need and/or time/money in the Asset Store to find the gameplay systems other people already made for you.
On a 1 year time estimate, you don't have too many options not to use third-party tools, so let me suggest Articy Draft or it's successor currently in development. This is a tool which intends to help you creating highly story driven games and it also comes with Unity 3D Integration you can use to design your game, story and manage your scenes and assets and after you're done, just export everything into Unity. It is not a “build my game” solution, you still have to spend some work to fix bugs and polish your game in Unity, but it'll make your life a lot easier and could help to reach your 1 year goal.
Good Luck!
