How to create custom media keys with unique functionality for keyboard?

My idea is to create unique media keys on the keyboard for:

1. Inputting shortcuts, e.g. replace Shift+Ctrl+S with only one key
2. Inputting symbols, e.g. emoji smiles
3. Inputting certain words with only 1 key

The solution must be universal, so any other person could easily use the keyboard, ideally only by connecting it and installing the driver. I don't want to override any of existing keys.

My plan is:

1. Reprogramm the keyboard controller, in order to react with non-standard non-taken scancodes. (Thus, keys won't work before driver installation, that is ok)
2. Create filter driver for Windows, which will transform those scancodes to shortcuts, symbols, words.

I've got a few questions so far:

1. Does my plan make sense? Are there other solutions of my ideas?
2. Are there any hidden difficulties in implementation?