MGE XE, the graphics mod for Morrowind which hooks into the engine and makes it do various pretty things, adds a system for grass and other groundcover, which is used by several mods. These mods come as .esp files plus meshes and textures. However, they absolutely shouldn't be loaded as plugin files by the game. They're only intended to be loaded by MGE XE's distant land generation tool, which then outputs some files which MGE XE uses at runtime. If you make the mistake of activating them as regular plugin files and then launching the game, you'll have terrible performance, and two copies of every blade of grass, one of which won't sway in the wind, and will likely have other issues, e.g. unwanted collision and rendering issues (as the distant land generator ignores/overrides certain Nif properties in grass files).

This basically means a modded Morrowind setup will likely have a bunch of ESP files which need to be in the VFS when the distant land generator is run, but shouldn't be in the enabled plugin list when the game runs.

There's basically nothing that clearly indicates whether an ESP is a grass file or a regular ESP, so we can't do anything completely automatically, but maybe we can do something which allows the user to mark an ESP as grass, and then change what enabling it means so that ends up in the [DLWizard Plugins] section of MGE.ini instead of the [Game Files] section of Morrowind.ini.

This would also be useful information to make available to plugins. The OpenMW export plugin could add grass files to OpenMW's groundcover list if it had some way of finding out that was the right thing to do.