i think timing is the essence but yes, going with instinct on a player is the one.. if you feel a player is going to improve in profit enough over time to earn you money... then go for it!
I had a few shares in Mbappe a while back.. I sold them at a small profit but last season was disappointed about a) the return i got on investment for the length it took and b) the returns i got in dividends (basically at that time i wanted to use that dead money on something else).
That was justified by a slow start to the tournament but as soon as he scored verses Argentina i jumped back on at slightly higher rate than i sold him for but am 1.50 per share in profit which for anyone is a decent return.
He was always going to come good eventually... but my point is you could get in too early and have dead money doing nothing which isn't really beneficial for you. By catching anyone at the right time as they come to form i think you could be in for some fantastic returns. The positive about Mbappe now is that he's so young he could dominate the papers for a very long time which is good news for all us holders!