A few of my immediate ideas. Apologies if the PR already addresses some of them, I tried reading through the commits but other than recognizing some keywords, I don't really know what it's doing 😛 Also some of these assume callbacks will be available per tile, which I'm not sure will be possible, but anyway:

• separate sprites for front and rear parts of bridge heads (absolutely necessary, IMHO)
• allow querying height above ground (e.g for solid viaducts when low, open arches/pillars when high). Could also allow pillars themselves to change graphics depending on height and/or location, so they can be wider at the bottom, etc.
• allow querying tile_class/groundtype (for snow-aware bridges, gaps over roads, etc.), also querying tile_slope to allow making pillars/bridge pieces look nicer on slopes.
• maybe allow querying date of construction/age?
• ability to define the underside as impassible/no construction allowed (possibly per-tile?), to allow for visual tricks like floating bridges, or the aforementioned solid viaducts etc. without having roads/ships/whatnot going right through it. Not sure if this one is possible or not, but would be neat 🙂

Transport types allowed (Road, tram, rail, ships, possibly filter on road/railtypes too, like no dirt roads on cantilever steel bridges or maglev on wooden trestles)

Possibly in addition to this, a flag to just disable the bridge overlay sprites (maybe not including tramtypes and/or catenary), rather than disallowing the whole road/railtype? It might be contentious with road/tram/railtype set authors, but would for example allow trestles with non-solid (see-through) decks to remain compatible with railtypes which don't normally provide nice-looking overlays for such a bridge.

Patterns for bridge tiles to use on different lengths of bridges

You may have had this in mind, but also being able to define "arrays" of layouts would be neat. So for example at certain lengths (in this example multiples of 11) a bridge could do [3-part-layout, 5-part-layout, 3-part-layout], but at other lengths just repeat [5-part-layout] or [3-part-layout] or whatever.

Also each layout should be able to be marked as repeatable or not, which would allow for both tile-able and one-off designs, or a mix like above.

There are some other ideas that are probably a bit too crazy (working draw bridges anyone? 😛) so I'll leave that out for now. But everything else here so far is +1000 from me 👍

Some bridge sets add types are only intended to be used in combination, not on their own, e.g. the Modular Wider Bridges Set. With more/richer bridge types, more GRFs may also do this.
It may be useful to add flag(s) which prevents towns or AI/GS from building bridges of this type (this is already implemented in my branch and in the Modular Wider Bridges Set).

On pillars, at the moment in my branch the presence of pillars can be set on a per-corner and per-edge basis for each bridge part, see here. This is currently just used for stations (which have corresponding flags for which pillars are not allowed), but it would make sense for something like this to also be used for road/track/etc underneath, as suggested above.

I'm a little surprised that there is any interest in developing the bridge
I have a suggestion, only 3 suggestions
1.Necessary
The cost of the bridge, the size is only 1 byte (up to 255), which is quite very cheap, so I want to redo up to 2 bytes (up to 65535). Because I want a miracle bridge to cost more than a bridge that is more than 40 tiles long
2.New feature "Hidden"
This is a useful feature to hide from purchases in the menu. Therefore, it is better to hide the Maglev and the monorail from the wooden bridge. And not only that, but I don't need bridges more than 10 types of bridges maglevs and monorails
3.New feature "symmetric x4" This is desirable.
So currently OpenTTD bridges are symmetric x2 (-0(23)(23)1-), for cable-stayed and suspension bridges is too short. It would be more interesting to have very long bridges. But only conditionally, by default, they build symmetric bridges x2, if you want x4 - you need to add the flag (x4 is symmetric). Table ID for symmetric bridges x4-up to F(up to 16).