Yeah that is a bit of loaded question because there is no “one size fits all” The ROV/AUV sector is very much requirement driven.
The biggest challenge however, in my opinion is and always has been the cost of these systems. The commercial sector is used to paying whatever price is necessary to do the job (> $1 million). Professional systems for the “smaller guys” (i.e. research and consumer) start at around $50,000 before you start adding accessories. Do these systems really cost this much in materials and labor to make? No, but that’s another discussion. These high prices are keeping tools out of reach for many who need them to do work.
If you really wanted to innovate, the questions you should be asking are, how do I keep the costs low while still having a high degree of functionality. Obviously there is a limit, but there are a few efforts out there right now to build systems for around $3000 that would have the same functionality or more as $50,000 systems. That is where I see the market going.
Of note, there is no low cost AUV as of right now. The lowest cost one coming out is this one from RiptideAS, but that’s still around $10,000 and still in the prototype stage.