With all due respect, Akshay isn't capable of making Dhoom 4 huge. Dhoom 3 has records in terms of opening, weekend, week and ect including Overseas and Worldwide. It was also the HGOTY even the biggest grosser in the UK to this today. So it would be impossible for him to beat all these records and the previous footfalls. Only Salman & SRK can make the fourth installment huge because they are bigger crowd pullers. Prabhas as well but I'll get to him in a min.
Ranveer is not a superstar level to make this franchise huge as the lead actor. Giving back to back Blockbusters was good but he has to keep pushing forward by giving more big grossers. He has the overseas pull but his stardom isn't incredible enough to beat Dhoom 3 previous records. Frankly, he just needs more time.
If they do consider casting Ranveer then he should play the cop. Prabhas would be a better choice to play the main antagonist because he's a big star with a pan-Indian appeal. His lineup is terrific and it will upgrade his stardom to another level if they work at the BO. I think this franchise needs to be rebooted although I will miss Uday Chopra because he was very funny in the previous Dhoom