I think we should keep in mind that Alexander's supposed plans to conquer Rome-Carthage come from Perdiccas. Its possible that Perdiccas exaggerated the scope of the plans to get them cancelled. His main goal was to consolidate his power not to launch new campaigns. I tend to agree with historians like Heckel who attribute Alexander's goals as to conquer Persian Empire only rather than world conquest. I don't think Alexander ever had any intention of conquering India his previous campaign was more about securing his eastern border region. Had he lived Alexander would have had enough trouble putting down revolts in the current mass of territory he ruled, such as the one Peithon faced, without adding new lands. In my view a more realistic option for Alexander is the planned expedition along the Arabian coast, this would have been profitable and not very costly. He could also subdue the portions of Asia Minor-Armenia still independent.