Since the 915 prototypes will likely be seeded in June at the AT&T National, it may be too late for major design changes. That said, this post can at least provide some perspective from better amateurs about the current state of driver technology.
The 913D2 is a fantastic driver for the player who wants a little more spin, but Titleist made a big mistake in giving the 913D2 and D3 similar launch profiles. Yes, the 910D2 spun too much so it was the right move to bring that down in the 913D2, but not enough was done with the 913D3.
Numerous anecdotes show that higher ball speed players get substantially more distance from the SLDR when they are fit for both the SLDR and 913D3, usually in the neighborhood of 10-15 yards. The difference is essentially due to being able to fit a SLDR that launches the ball much higher and also with lower spin. I have a 165 mph ball speed which is nothing special, about PGA Tour average, and Flightscope's trajectory optimizer (http://flightscope.com/products/trajectory-optimizer/) shows my current launch conditions of 14* and 2400 rpm spin produce a carry of 286 yards and 297 yards total with medium course firmness. However, with a launch of 18* and 1500 rpm spin, I get 294 yards carry and 308 yards total for a difference of 11 yards. This difference doubles when course firmness is set to hard. Unfortunately, 18*/1500 rpm is simply not possible with my 8.5* 913D3 unless I make a completely unnatural swing.
If we agree that higher launch/lower spin is the goal, we want a low/forward CG as in the SLDR and Titleist 905S. Speaking of the 905S, it was a driver ahead of its time, a dark age before Flightscope or Trackman. The 915D3 should at least move the weight chip near the face as in the 913Fd. The face could also be deeper, and even if that meant making the driver 460cc, players wouldn't worry because workability wouldn't be affected. A forward CG reduces MOI and therefore "forgiveness", but that's not the kind of forgiveness that better players need. During a range session this weekend, I took note of my ball impact locations. Even my absolute worst shots, i.e. push fades and snap hooks, were hit in the middle of the clubface. The shots were bad because of poor face angle and club path, things that no club can fix. The kind of "forgiveness" better players want from off-center hits is to not lose distance; we want consistent ball speeds across the face, especially above the center of the face where we can maximize the chance for a high launch/low spin shot.
As for the 915D2, more of the same would be great. I'm a Titleist loyalist, through and through, and I love my 913D3, but I'm not getting paid to play any particular brand. Unless the 915D3 offers higher launch/lower spin, myself and numerous other Titleist loyalists may find ourselves playing SLDR 2's next year.