I really don't see extra batteries in the Blacktip future. They make bigger scooters that hold more batteries. The body diameter is such that you would be doubling the body length to add another set of batteries. Design wise just doesn't make sense.
But I do think a couple extra inches in length, move the batteries forward, fix the trim. That is a very easy upgrade that can be done with minimal work. Maybe something new for the next model year? The only thing I really find a lot of complaints about is the ability to trim.
There are dives I would like to do where I think 2 x 12Ah batteries are not enough. But, they are close to enough. The biggest issue is not wanting to plan a dive based on using ALL my battery capacity. So, a 2 - 2.5 hour dive would require all of 2 x 12s, and leave me with no reserve.
I think, also, if I were planning a dive that needed 2 hours of battery, that probably means a dive where I am going to be on the trigger pretty much the whole time. And for that, I don't care AS much about the trim. The BT trim is really only an issue (to me, anyway) when stopped. While actually driving, the only time I notice it at all is when turning and then it's not that big a deal. Caveat: That is based on using my tow cord at the 6/12 position and using a Lark's Head knot to attach the bolt snap to the tow cord. When I tried it with the bolt snap sliding freely on the tow cord, then a little constant pressure was needed to keep the nose from flipping up while driving. I have not tried the tow card at 3/9 o'clock, but I imagine that would be a nightmare, with the way the nose wants to flip up.
So, to ME, what would be pretty ideal would be a tube or tube extension that added 2 battery sockets and allowed you to use 2 batteries or, optionally 4. It would be the right length so that if you used 2 batteries, it would trim out. If you used 4, then it would be tail heavy, like it is now.
That would give everyone the trim we want with the current battery capacity. And, at least for me, give me extra battery for when I want that - which, for me, would generally be dives where I'm on the trigger the whole time and not concerned about how it trims out when stopped.
Also, that would make it where people who don't mind the trim of the current unit, but want longer run time without shelling out the big bucks for 12Ah batteries, could put in 4 of the Waitley 9Ah batteries. They would get double their current run time, for relatively cheap - as long as they don't mind still having it be tail heavy.