How to wish a sick person to recover from the illness? I'm interested in the expression for informal usage.
The context is important. How sick is the person? Just a mild illness or a serious, life-threatening disease? How you write something will vary greatly depending on the answer.
For a mild illness, I would say something like
I hope you'll be feeling better soon.
For a serious illness, where you know the person may not be 'feeling better soon', I'd say something like
I'll be thinking of you and wishing you all the best for a swift recovery.
I would definitely NOT say "I hope you'll recover" as it carries the implication that the person is likely to die!