Hi Jacc,
If you have some cell with data validation with a list all you need to do if you want to write there whatever you want is uncheck error message in data validation settings.
Data validation - error alert.png

Not sure if this is exactly solution you are looking it's just first idea that come to my mind.

I made some videos about data validation, maybe it can inspire you
https://www.youtube.com/watch?v=IKAFYZ2u4Og
https://www.youtube.com/watch?v=z_rMZV8zHPg

Hope this help.

Best regards.