Big Edit: I realize now that my infernal dragon section needs a kinda big change, and I'll document it here: Instead of a % increase damage to champions (which would massively buff tanks) the infernal dragon should give % increased damage based on bonus ad and ap.
What this will do is allow the % increased damage to scale with the damage dealers it was meant to scale with without buffing any of the areas I mention that the dragon buff shouldn't be buffing.
I agree that Cloud Dragon is too weak. Well, more specifically, its power is very difficult to understand and I don't think it has enough of am impact when compared to the other dragons. It isn't worthless (at least 3 of them aren't, kappa) but whenever I get one my first thoughts are 1. At least the other team did not get it, 2. More true damage for that elder dragon buff, and 3. I can get outta base faster now.
It does need changes, I'm just not sure your suggestion is the right idea; Riot just removed boot enchantments because the only one being purchased was the increased move speed one. Their reasoning was that the general increase in movement speed was resulting in more skill shots being missed than was intended, etc so they wanted champions overall moving slower again. Your suggestion would basically revert that. Something is definitely needed to make cloud a little more attractive, but that isn't it. Maybe slow reduction, but that could get out of hand; I also wouldn't mind some tenacity, that stat is REALLY hard to come by these days.
Also, if the two dragons that need nerfs get them, its possible that the cloud dragon will actually end up being powerful enough as it is; its problem right now could just be that it is getting overshadowed by better, unbalanced options.
Ocean Dragon is overpowered at the moment because you always get the heal. The pbe nerfs to it (making it so a slightly adjusted heal occurs only when you haven't taken damage for a certain amount of time) will really help to bring it in line. I don't have much more to say about this one; Riot already dealt with it.
The Infernal Dragon is completely overpowered and needs to be changed. It is suppose to be the dragon you take when you want better combat vs champions. At least, that is what we were made to believe. And that is certainly what increased ad and ap does. But it also increases your minion pushing capabilities, your jungle clear capabilities, tower pushing, and epic monster killing potential. This dragon is literally a jack of all trades dragon.
To make matters worse, in addition to it giving its slayers a whole bunch more power than just killing champions, it isn't even particularly effective at doing that. On hit build champions don't gain much from the Infernal Dragon buff because they aren't building a lot of ad or ap to deal their damage. In addition, additional ap makes support champions and healers more effective at keeping their allies alive/ providing sustain. So basically the dragon is failing at doing what it is suppose to be doing while giving a whole lot of free power in areas it shouldn't be. It, by itself, is overstepping onto ocean dragon and mountain dragons roles.
To fix that, I suggest that the Infernal Dragon be changed to 8/16/24% (or whatever numbers are deemed appropriate) increased damage to champions. This way, it isn't giving random power to champions that snowball leads terribly and, at the same time, it is giving an appropriate power boost to champions building items like witts end and sheen items who's power isn't amplified with the current Infernal Dragon.
I actually really like the Mountain Dragon and don't want it to change at all. With 2 of these babies, I was able to solo baron playing Shyvana with one (okay 2, almost forgot about the jungle item) lifesteal items and come out above 70% health. Its damage to towers is also very noticable and its in a way that is more fair than the previous second dragon.
The previous second dragon had the problem of making anyone a good splitpusher due to the damage not being tied to the damage output of the champion. That is (was) straight overpowered. The point of the dragons now is to amplify the strengths of a team and fight/contest dragons your team wants over others. If you have a teamfight team, for example, mountain dragons are less appealing because by the time you guys are hitting towers, you have ace'd the other team (making that extra damage not useless but less important). As opposed to the jax or teemo that stays top all game threatening objectives (where mountain would be much more useful).
If infernal dragon were to be nerfed, I could see no changes being done to the mountain dragon. I think it is just fine the way it is.