I think the Runic Echoes nerf that is on the PBE currently should move Gragas more towards the Cinderhulk build, but not enough that Runic Echoes won't be used on him. Runic Echoes is a little bit overtuned as is almost purely based on the insane movement speed it gives in the early game. Any jungler that can take advantage of the item is going to because it gives them so much more map movement over other builds and it also speeds up clears and makes early ganks more damaging. On top of all that, it gives mana sustain, something tanks have trouble with early on before they can build any mana.
What is being sacrificed is a good chunk of health for the midgame teamfights (~500 health total at midgame), but as games are designed to be more snowball oriented this season, the sacrifice is well worth it for a lot more pressure early on. By all rights, Runic Echoes is just a better item right now. If the meta shifts or buffs/nerfs happen to the jungle items, I would imagine AP tank/fighter junglers might drop RE as a first item, but if you can use it right now (
etc); I think you have a lot of reason to.
I think they need to bring Warrior and Cinderhulk to a level closer to Runic Echoes. Right now, Warrior purely gives stats, good stats, but it doesn't give as much as Runic Echoes does right now: mana sustain, extra burst damage (low scaling), faster clears, and faster map movement. You can actually see a problem with AD fighter junglers in particular in that they are struggling in the current meta. The only Warrior junglers you see have high success are ADC's, and I think there is a problem there.
Warrior definitely needs a rework to bring champions like Jarvan, Vi, and Nocturne back into the jungle in preferred buildpaths as they simply can't compete with many of the AP junglers that are strong right now due to Runic Echoes. Riot stated that diving fighters were on their radar and were edged out by the Mage Update happening later this year. I think a big thing that could help these types of fighters would be a change to Warrior Enchantment to benefit melee fighters a little more than it currently does.
As far as Cinderhulk is concerned, I think it is pretty good for champions with high base damages, but I think it is more situational than it used to be for champions that could opt for Runic Echoes. Basically, you get Cinderhulk if you REALLY need a damage soaking champ on your team. It might be about as good as Warrior on AD fighters due to them usually stacking a lot of health after their jungle item and with items like Titanic Hydra, Sterak's Gage, Maw, IBG, Frozen Mallet, and Black Cleaver, they can get their offensive stats elsewhere if they need them while also increasing their tankiness.
I don't know what they can actually do with Cinderhulk other than a possible small buff to its damage to bring it in line with Sunfire Cape. I don't think it will cause Top laners to opt for Smite like they did before as the jungle item is more expensive now (about as expensive as Sunfire Cape) and Challenging Smite is not as strong as it used to be. Otherwise, I think the item is fine for a large variety of champions and if anything, it just needs a little fine tuning in terms of clear speed and champion damage to make it more competitive with Runic Echoes.