Of all the 1D members it's obvious Harry has the most solo potential BUT even with that I don't see him reaching the heights of say JT, he may come hard woth his first era but I don see him doing well in the long run.
Also he needs to stick to POP. If he takes an alt. R&B route it'll be played out. Zayn switched to R&B, Nick flirted with it and JT made a career out of R&B inspired music, at this point it's become a cliche. He needs to put out solid pop records, get involved in his writing and image and bank on his status as a sex sybol, atleast to his core fanbase.
I also think he should try to do a bit more mature music than 1D, that would get him some more casual and older fans. JB did a great 180 with 'Purpose' and I can see Harry doing something similar.
Just my two cents