Runes provide a boost to a stat that is typically overshadowed later in the game by items. While super useful early and mid game, depending on type, they don't have as significant of an impact once you get a few items built. Lets say an ADC has pen reds (or flat AD) and armor/mr blues/yellows. Make laning easier, but how much is 15 bonus armor going to help late game? It helps, but not a lot. Same with the ad marks, a nice 10% damage boost at level 1, but barely 3% with 3 items and a few levels. (No, I didn't run numbers there, just guessed a bit, but you get the idea)
Gold generation, though, is a significant boost at every point in the game. Well, at least until you hit full build. More gold directly relates to better items, which leads to other runes being overshadowed at an earlier point. Be wary buffing this.
I'm not against buffing runes that we don't see used often, but the impact of even a slight increase can dramatically turn the tables. This is a post telling of caution in the numbers, the hidden bits that you don't see until they're so broken you have to use them. Auras, anyone? Similar issue, to feel impactful they must be large enough to be seen, and by then they are so broken that they are oppressive.
I think it would be interesting to get a blog post regarding the use of different types of runes (by lane, build, whatever makes sense). It would help identify which runes are not used at all. Health regen, perhaps mana regen, and perhaps crit damage might need to be looked at, but are there some champs who exclusively use these runes (thinking vlad and morde perhaps for health regen?) that would become unbearable if there was a buff to the numbers?