Ok made up a simple example with some platforms more in "the Spryke way"
(well, if you use a good amount of imagination to see awesome graphics when there's now crappy sprites XD)
and some random 3D objects placed here and there:
Please login to see this link.
Warning: default Fusion platform movement, getting stuck is behind the corner XD
preview gif:
Please login to see this picture.
A very plain level, with some random ideas like a "trap" platform rotating down towards the end
(I think it's a nice fx because you don't have to animate it, and so it's flexible to behave the way you want)
A random "platforms field" of 100 elements is created towards the far right of the level, to test performance in crowded environments.
There's some displacement maps, I found them damaging performances with "resample" ticked, but maybe it's only my machine (see performance below)
The way it is now all the "3D" stuff is behind the top layer, where the player and 2D platforms are.
With some little customization, some 3D stuff could be put even "above" the player/platforms.
I've left P3D simple "debug" box open on topright of screen so you can enable and disable "resample" from there,
and also change render depth (how far you can see) with +/- buttons.
Only the tiles where I've written "2D tile" are the real flat 2D obstacle platforms,
against which the 2D hitbox collides as it would do in any plain 2D game.
You can see the trick by using Q/E and/or A/D keys, those will tilt camera X and Y angles,
so the 2D platforms will go out of sync with the 3D world, to resync again press "enter".
On the editing process:
I won't say this is exactly that comfortable to do on a large scale with plenty of platforms XD
but don't know either how much hassle is making those "pseudo-isometric" platforms by hand...
spent a couple hours making up the short example you see, though I can probably improve some aspects of the process.
So basically I think it's worth considering not as a "time-saver" solution, but only if the visuals are pleasing enough
(on that side, I guess the shaders could be slightly modified to improve on the antialiasing side, but should check better in case it's needed!)
On the performance side:
the file is capped at 200 fps, on my not that good pc it runs at solid 200 fps when not scrolling, at 180 when scrolling
worse performances with "resample" ticked when the "displaced volumes" are in sight, going to about 130 fps
and when there's about 100 platforms in sight (far right on the example), going down to 150 fps.
When going full screen (alt+enter), it's almost always at full 200fps, but our good old microstutter is more noticeable here..
There are some other things to consider, possible issues with the depth buffer and resolution changes etc. (you could probably avoid the depth buffer though)
but won't go too much in detail as you might not like the result anyway.
If on the other hand you like the idea, feel free to ask more info,
I've spent some considerable time working on this kind of stuff and I'm happy to help Spryke if I can,
even if you think you want to do it and end up not using P3D for that ;D