A couple of feature requests

[Astrobe]

Let mods trigger the flashing borders effect
This is the visual effect that happens when the player is hit; a brief red flash on the borders of the screen, with control on the color, and ideally the duration.
Use cases:
- a green flash for healing
- "buff"/"debuff" effects such as seen
- I have a case where incoming damage is nullified, so the red flash doesn't happen, but I would like to have a visual effect (like an orange or blue flash) to warn the player (audio isn't reliable as some players don't play with sound, and particles don't work very well when the player is moving).

Could be done using the same technique as the vignette mod, but generalizing the existing engine effect would be more efficient.

A new texture layout for plants
A 3 textures layout aligned with the x,y and z planes.
Use case: make leaves similar to the bushy leaves mod.

Engine support would probably be more efficient.

Let mods cap the view range
Let a mod override the view range set by the user, and if possible an easy way to restore the user's setting (e.g. by setting the cap to a negative value).
Use case: simulate atmospheric effects such as fog or heavy rain.

[cx384]

Let mods trigger the flashing borders effect

This is already possible. You can show hud elements (also transparent) in minetest.register_on_player_hpchange(function(player, hp_change, reason), modifier) and remove them after some time.

A new texture layout for plants
A 3 textures layout aligned with the x,y and z planes.

Doable with node boxes.

Engine support would probably be more efficient.

Let mods cap the view range

If you really want those features I suggest to create issues.
Otherwise it is unlikely that the minetest core devs and contributors will take them into account.

[Desour]

Let mods trigger the flashing borders effect
[...] generalizing the existing engine effect would be more efficient.

It's not like anything can magically be made significantly more efficient and better if done in the engine.

The issue you're probably having is that if you implement the effect with the current HUD API, the effect is not coming with at the same time as the damage, and it all feels somewhat sluggish. SSCSM should improve the situation here, when it will be added.

A new texture layout for plants

Can be done with a mesh or nodebox. If it's slow, those features should be improved. Idk if there are enough use-cases for your suggested drawtype.

Let mods cap the view range

Lucky you, there's already a PR for that, and it's likely to go into 5.8: https://github.com/minetest/minetest/pull/13448

[Astrobe]

It's not like anything can magically be made significantly more efficient and better if done in the engine.

The effect already exists in the engine, but hardcoded. It is kind of dumb to remake a poor man's version with a script.

A new texture layout for plants

It's not like anything can magically be made significantly more efficient and better if done in the engine.

Can be done with a mesh or nodebox. If it's slow, those features should be improved. Idk if there are enough use-cases for your suggested drawtype.

It's not like my mouse slipped and I accidentally inserted a link to a random mod. It has been done with nodeboxes, it is hack-y, and it does have performance issues.

I'd rather ask for a new layout, as it is a totally legit use case for this property and the docs says that there's RFU bits for that, than ask for improving nodeboxes - which is not exactly what I want and which for sure would be done anyway should the opportunity present itself.

For uses cases, there are as many as for any of the existing layouts. Perhaps more, because you never know what people will do with features. The firelike layout has been used for cobwebs for instance.