I don't think a single 3/4" sheet of plywood is enough for a stone floor. Stone requires very little deflection.

So is there a joist under that bit of sub-floor? All joints in the sub-floor need to land on a joist. For stone I think the usual requirement is L/720 for deflection and on 2x10 jpists I've seen people put down a couple of layers of plywood 3/4" and then 1/2" to give the 1 1/4" thickness followed by a crack membrane like Ditra or Nobelseal TS. When I did my floors the 3/4" planks were left in place and rescrewed, then 1/2" plywood put over the top followed by self leveling compound and Nobleseal TS before ceramic tile went down.

The sub-floor requirements for hardwood are very different from those required by stone or marble and so there is often a height difference at the transition. You should not lay the stone on anything less than the industry standard. Some pros should chime in.

I know that would work - maybe someone who does this for a living can give you a thinner solution - but it won't be much thinner. Make sure you stagger the seams on the plywood.

From the picture - By the hardwood floor - it doesn't look like there is any joist for the subfloor to rest on? You need to have the edge of the plywood subfloor to be supported.

Ditra won't help - that's for crack suppression.

Bad move. would not have cut the original underlayment out, and if i had to i would have done it methodically. youve shown just one pic.

Now in at least the vented/joist bay from what i can see in one pic youve left yourself in a pickle in regards to getting adequate cross blocking in. Every single seam now requires blocking that includes every perimitier seam at joist bays. The one exception being T&G joints in your new initial underlayment(field only @ 16.OC which likely needs to be double 1/2" exp or 3/4" both plus a membrane. No cement board.. Youve also not supplied nearly enough photo and structural info to make a full asessment for a smart , approved plan to install stone over.

How far back is the joist under the hardwood floor? If it's a couple of inches - maybe you could scab on to it - to support the edge of hardwood and your new subfloor. Is there a basement underneath?

2 more pics, the first picture shows where I started scabbing 2x4s for extra screw support for the subfloor. Is there a certain part you need me to take a pic of? I realize it may have been better to just leave the original subfloor but some of it was rotted from cracked tile, plus easier access for moving new plumbing around. I appreciate the help so far. Fhe 2 walls that are the are just dividing the hall and bedroom and not load bearing if that matters at all. You can see how they cut out a lot of the old 2x4s for the old plumbing etc.

IMO you are going to need a 2x10 running parallel to the duct to pick up the edge where you cut and the new plywood that you lay. What are the end of your joists connected to? Also I don't like the gap in the bottom plate of your wall. Maybe there is a professional who has run into this before and has a good solution......I might consider removing the floor back to the middle of the next joist so you get a solid nailing surface for the new plywood.

Why not just cut the floor back 4.5 inches to the 2x10. I will take some careful circular saw work, but then you can expose half the top of that 2x10 and attach your new subfloor to it.

forgot to post the pics

didnt post

2x10s 16" on center with 3/4" and then 1/2" ply and a crack membrane like Nobleseal TS if you don't want the added build up of Ditra would be a good foundation for your stone.....but you need to get the end of the ply onto the top of a joist. I don't really like the idea of joist hangers and 2x4s between the joists as it's going to get cluttered fast and there's lots to keep level and I'm not sure if it will meet spec. The professionals should weigh in on that, but I think the bottom line is that if you are replacing sub floors you should always take it out to the middle of a joist and not between joists so you have something to rest the new floor on. Removing the hardwood and taking the floor back to a joist might end up being less work than all the 2x4s......

Putting shims on the joists might get frustrating depending on the way they are off and you need them to be continuous so the plywood is supported everywhere. If one joist is 1/8" lower across its length than the next one to it, then a single 1/8" thick furring would work, but it might be easiest to use leveling compound and just crew the ply down tight to the joists you have. Again get some professional opinions before you do this.