I just love those 1-pixel high path blocks, and I'm glad someone's brought them back. Other than you, only Crayfish's Funiture Mod that I know of nowadays, and only one type? I might be willing to mock up some textures if I knew the req's, though I'd be little help in proper file formats and such.

I've talked a bit with the folks at Charm, they seem less enthused about the idea, but since you already have paths?

Anyway. I'd like a bigger variety of materials/variants. Adding in button or pressure plate variants like you do with carpet would be really cool. Some ideas:

• Types of ground cover: Roots from Logs? Other ground cover like Leaves, Sticks, etc. but those exist elsewhere. Dirt, sand, soul sand, or gravel trails 0-high might be really cool.
• A few more decorative stones, basically for each stone variant / brick type / carved stone pattern, would be really cool.
• Sandstone. Purpur. Prismarine. Endstone. For mods that extend these to bricks and such (Like Quark's Midori) optional variants would be nice.
• Wooden Plank walkways would be great. Especially for bridges, on top of fences, etc. Maybe something like single Beams for stripped logs.
• Various Terracotta/Glazed tiles would be awesome. You could go crazy with the patterns, too.
• A carpet edge a couple pixels from the side - patterned, straight, or fringed - would be really cool. Especially if it connected to neighboring carpets... I guess that's pretty much like Chisel's Connected-texture carpets.
• Glass panes would be a welcome addition, and fit naturally here. Better than in the middle of the block; hopefully it would conform to the block below, so if panes are present it would only cover where they do?
• An Iron grate fits in well here in the same vein.
• Heck, some bling from gold coins, treasure, and/or gems, would be great.
• Snow already exists, but what about frost or ice? 'Puddles' of water, mud?
• Edit: Skins / Furs from leather?

Would it be crazy to have an under-side block-type for carpet-like blocks? Wooden support beams, roots, or bottom glass sheets, for example. For most I'd just flip the texture. 'Hanging' or side variants would probably be too much.

The second main suggestion/request:
If it doesn't diverge too much, other path types (like Grass Path, 15 pixels high) would also be a very welcome addition, as well as half-slab variants so you didn't have to jump everywhere on paths or put in mismatched blocks. (i.e. 7 pixels high for consistency). Not sure how you'd get them; would you craft them, or shovel/tool them again, or what?

A couple mods used to do this, (Pathomania, Simply Paths) but they kind of suck without at least a slightly differing texture. 1 pixel height difference isn't enough I think. You could really make them out of any natural full block in the game, but here's a few more ideas:

• Other grass types: Packed Dirt from Dirt, Mud from Podzol, Stony Dirt from Coarse Dirt, ?? from Mycelium. Cracked Sand/Sandstone? Stony Gravel? Cracked clay?
• Natural Stone Types: ...Not sure here what to do on these. But cobble, stone, and stone variants should apply. Maybe just a color change; or maybe like a 'worn' state?
• Stone Bricks and Bricks - You could get pretty creative with all the different stone patterns/variants.
• Wooden planks and such I'm not sure what to do with, but it seems a natural fit.
• Prismarine? Netherack? Nether Bricks? Soul Sand?
• Snow and Ice: Packed Snow? That awful slushy stuff?

This one meanders a bit, sorry. There's probably enough in there for a separate mod. I wouldn't be mad at that lol. If only I had the coding chops for that. I just love my paths. Thoughts?

...Would this legit be better as a separate mod? How might I get development support on that, say if I was doing the art?

I'm noticing looking over this that there between the two categories, they don't have a lot of overlap in materials that work naturally.

So really, I'd probably put the top-blocks and shoveled blocks as separate categories. Stone, wood, and the like work well for top blocks; whereas granular blocks like dirt/sand/grass/snow/gravel work best for path-like blocks.

Sorry for taking so long to respond, forgot I did not respond to this yet. I'll leave comments on individual points:

• I want to avoid adding random cover blocks for the sake of adding them. Other mods add that sort of thing. I mainly want to stick with brick path like blocks. I do like the idea of vertical glass panes and iron bars however, in the style of carpets.
• More path types is something something I would consider, though as you said mods exist that do that sort of thing. I will consider podzol and mycelium, but probably not any other types, especially not stone types.

Overall, you do have enough ideas if you wanted to start your own decorative blocks mod. Inspirations really tries to err on the side of vanilla, meaning I avoid adding too much of one feature lest it become over saturated. Hence why I originally added only 4 path types.