Suggestions for building plywood landing and connecting to concrete stoop?
I would build it more or less exactly as you've sketched it, but on posts like a deck. Don't attempt to suspend it on the block wall. Build a conventional floor system with 2x6 sides and joists on 16&quot; centers, then cover it with 3/4&quot; plywood.
Drop six legs to the slab fastened to and supporting the rim joists. I'd probably use one 2x4 under the rim and one against it and lapping onto the joist, inside. Be aware that these will vary in height to accommodate slope in the slab.
Post detail:
 __ __
|  |  |
|  |  |&lt;-- rim joist
|  |  |
|  |__|
|  |  |
|  |  |&lt;-- outer leg member
|  |  |
| &lt;|--|--- inner leg member
|  |  |

You can then fasten it to the block to hold position, but you're not relying on that connection for load bearing. You could also just put a dab of construction adhesive under each post to lock it in place.
As to the slope in the concrete stoop, I'd consider twisting the deck to go from sloped at the concrete to level at the other end. It really depends on what the user prefers, though. The alternative is to set the deck level with the low point on the stoop and create a small transition wedge to blend with the concrete.
A good railing strategy is just to use 2x2 spindles on 5-1/4&quot; centers with a 2x4 top rail (oriented vertically). Optionally cap that with another 2x4 or 2x6 to create more of an elbow rest. Fasten each spindle to the rim joist with two 2-1/2&quot; screws.
The stoop step is a conundrum. Your idea to build a wooden bottom step has merit, but stair sets with random rise heights are a problem. I'd consider building a full 3x3 landing 7-1/4&quot; down from the lower tread, leaving you with two uniform rises above the landing. Mentally this is less likely to result in stumbles.