"Simple" meaning that they match the original look, just in higher resolution. I haven't seen any outside of improved trees and flora series.

I do not know of any since I really do not use any texture mods unless it adds new things. It has been awhile since I last played. I did do a check on Nexus so you might want to take a look at these.

I do not know how much changes if any Qarl's texture pack does since I have only tried it once a few years back and I did not use for long. http://www.nexusmods.com/oblivion/mods/18498/?

The other three I have never used so I can state how much of a change they add.

Enhanced Vegetation - http://www.nexusmods.com/oblivion/mods/23783/?

Oblivion Grass Overhaul - http://www.nexusmods.com/oblivion/mods/42400/?

Oblivion Trees Overhaul - http://www.nexusmods.com/oblivion/mods/46091/?

I've seen those but they're not quite what I'm looking for unfortunately. qtp doesn't affect the flora, and enhanced vegetation improves the distant billboard textures, but not the ones up close. The grass/tree overhauls remake the trees and grass in a different style than vanilla.

This mod comes close to what I want, but the author hasn't gotten around to doing billboards and has only done some of the trees so it's not quite there yet - http://www.nexusmods.com/oblivion/mods/45521/?

Apparently Vurt did a flora overhaul for oblivion at some point, but it doesn't seem to exist anymore so I guess it was never completed or had some other problems. Aside from that it seems like no one else has tried making hi res versions of the original trees and grass.

I think the improved trees and flora series is pretty faithful to keeping the vanilla look. That's what I use.

Agreed, I use them too and they look fantastic. I just wish the author would come back and do the rest of the flora along with the tree leaves lol