I want to make a pack that replaces the default tall grass, with a 3d model with realistic looking grass. It'll need to be at least 128*128, but probably more like 256 for it to look smooth. Does this sound practical? Is this tutorial here still useful, despite its age? Thanks.
the "normal" way that the game does models for grass and plants is by crossing two 2d textures over each other. This is fairly simple and works quite well. I would recommend keeping this model but just re-doing the texture to a 256 p realistic looking texture.
It IS possible to re-do the model and have it be 3d but for a 256 or 128 texture this would be take a lot of time and if you're not already a texture pack artist would end up consuming a ridiculous amount of your time if you were to do this yourself.
Think about why there are virtually no resource packs already like this. You want to create a 256 x 256 model which is essentially like building a 256 x 256 block landscape in minecraft ONE pixel at a time with some mc model creation software AND THEN make a good looking 256 x 256 texture for it AND THEN write the model file for it. All for one texture? Probably not worth the time. If there's a very specific or crucial reason for it then maybe it is. That's up for you to decide.