So this is somewhat of a re-post since the last one got zero comments/replies.
When working on some wall designs, I really dislike how I couldn't combine Stone Brick Walls and Iron Bars without a funky gap between them, making it look really awkward instead of a nice chain-link-fence-style look. So, we have 4 different types of these form-fitting blocks, but they only form-fit to the full side of blocks (e.g. backside of stairs) or the same type. You can mix walls together, fences together, panes together... and there's only one kind of bar (maybe also suggesting gold bars?). However, as I've found above, there is zero mixing of multiple types. As I've said in the original thread, make it so that it is affected by which block you place it on (similar to a torch or hopper). If you place a non-wall form-fitter on a wall, it will try to connect to walls in addition to its own type (i.e. fences will still connect to fences, etc.).
Next up is a use for Shears/Pickaxes/Axes/something for glass panes. When right-clicking on one of the connections with the correct tool, that connection will disappear and will leave a small gap between the central "pole" and the block it was connected to. Great if you want a background palette, but don't want your wall/etc looking all messy with excessive clinginess to its surroundings. Also helps if you want a pole next to a building, but don't want the pole to cling to it. This ought to deduct from the tool's durability, like creating dirt paths do with shovels.
Third, walls/etc will tend towards a drop-off if the other side does not have a drop-off. Think of a bridge, and on each side the walls/etc will cling to the edges so that there's more room on the bridge itself and no space on the outside. Also, at a corner of a platform, the corner "pole" will try to tend towards the corner. If there are occasional holes next to the wall/etc, this won't affect it if both adjacent walls/etc are against the edge (alignment takes priority over placement rules).
Lastly, fix stone brick walls (and variants of course). They look absolutely terrible, because the tops could simply be solid bricks instead of looking like a 10-year-old just learned how to crop a texture. Particularly noting that from the side, the actual wall portions always have a solid brick on top (and half-bricks on the bottom that complete adjacent half-bricks), the top should also reflect this (the bottom side can also reflect always being half-bricks).