I wonder if you could render 3D assets into smaller 2D assets that could be fitted into a 2D tile set. I think smaller devices such as mobile phone, and mobile games are less than a 1024 by 1024 space, so your pixel resolution is limited. Some detail may be lost in the process.
I have an example for you:
This tree was originally rendered out to 3k, by 3k. Camera position is set to it's side so it will only render detail from the side, and thus imitate a 2D asset.
Please note: This tree is a preexisting asset in my visor. I did not create it, I only rendered it to give you an example.
This is only an idea. I have no idea if it would actually work. You might be able to modify such assets rendered in 3D to a 2D image format such as BMP, PNG, TIF, or PSD in whatever program you use. Using a software like Photoshop, or Gimp would also be useful in reducing the size of, and modifying any special effects of 3D rendered 2D assets.
Other objects like terrain, structures, vehicles, weapons, and character or NPC assets might also be rendered in 3D, and then added to your game as a 2D asset.