The OP put forward so many questions:
This cunning man decides to ask for infinite wishes.
Would God fullfil his wish?
What are the consequences of such a wish?
And why hasn't anyone (for example evil Ravan) never asked for such a boon?
Answer to these questions will be a little bit lengthy and complex also.
Sri Krishna says in B.G., as follows:
तं तं नियममास्थाय प्रकृत्या नियताः स्वया।।7.20।।
Being robbed of their wisdom by innumerable desires [and] being controlled by their own nature, persons take refuge in other deities by following one or the other religious regulations.
अन्तवत्तु फलं तेषां तद्भवत्यल्पमेधसाम्।
देवान्देवयजो यान्ति मद्भक्ता यान्ति मामपि।।7.23।।
But, that fruit of those men of poor intellect is finite. Those, who perform sacrifices, aiming at the gods, go to gods, and My devotees go to Me.
Apparently, very easy to understand, but these words carry very subtle meaning. Remember that these words have come from a Yogi of the highest order.
A few issues, to the extent of my understanding, can be discussed.
Consider the sloka 7.20 from B.G., mentioned above.
Being robbed of their wisdom by innumerable desires ......
Don't conclude that the Almighty God is a Fool enough to grant boons, without any controls? Never does it occur. A boon asked by a desire carries its own seeds of destruction.
However, brilliant a person like Ravana might be, the Creator will think many steps ahead of him. It is for sure.
The devotee shall get his desires fulfilled, but with some riders.
How brilliantly, Hiranyakashipu asked boons Brahma, for avoiding death, but ultimately met his end in the hands of Nrisimha.