There's been a lot of talk about balancing Elementals, and blizzard has already tried a few changes in in the short time they've been out - moving rag to tier 6, nerfing Garr's attack, moving djinni back to tier 6 after a short stint on tier 5. However, while these changes have hit these individual cards pretty hard - Rag and djinni went from strong to situational at best, and garr went from bad to unplayable (also putting us in the awkward spot of Elementals having the most tier 6 units with none of them actually being powerful) - the tribe as a whole continues to roll along as a late-game juggernaut that only niche or comparatively more difficult / expensive comps (eg dragons) can hope to compete with.

The problem

While some would point to Nomi as the culprit, this is only part of the truth. I believe the problem can actually be traced back to a much simpler card - Sellemental. It's too efficient at enabling the elemental play style, too strong of a standalone card in the early game, and too good at facilitating Elemental/other high rolling through easy triples.

Too efficient at enabling scaling

The idea behind Nomi is similar to hand buffing in constructed - you sacrifice a few turns of strength / tempo in order to translate it into strength later. This playstyle is balanced around the idea that the large future payoff is delayed, and you run the risk of losing before you get a chance to reap the benefits. For Nomi this trade-off is steep - unlike a card like brann you are scaling minions you don't even have yet; as such, it makes sense to having a larger reward since the chances of making it there are diminished.

The problem with sellemental is that it too good at fueling this, and drastically shrinks the delay between immediate weakness and later strength. To see why, try thinking of Elementals not as minions, but as effect triggers that each cost 2 net gold. For Nomi to work you want to buy as many of these triggers as you can as quickly / cost-effectively as you can - for the most part you don't care what minions these triggers actually are until you've already succeeded in scaling. Cards like stasis Elemental, which gives you access to another trigger to buy, or refreshing anomaly, which makes it cheaper to find more triggers, are especially good at this. Sellemental blows them out of the water: instead of two gold per trigger, you get two triggers for one gold - 4x as cost-effective, also without having to spend extra gold rolling between triggers. To put this in perspective, it's the equivalent of if stasis Elemental let you buy the extra minion FOR FREE. Alternatively, it's as good as Tavern Tempest, a tier 5 minion, while being almost twice as frequent in the tavern.

While I focused on the follow-on effects to Nomi, the same argument around effect triggers also applies to cards like majordomo, party Elemental, and garr, fueling their scaling with equal efficiency.

Too strong as an early buy

Sellelemental is also one of the best stand-alone early cards in the game. It's essentially a token start - already powerful for the extra economy - but is clearly better than Alley Cat and can compete with murloc tidehunter has the overall best card on the tier; you trade early vulnerability to 2/3's for a stronger overall stat line on turns 5 . 

(if someone who has access to HS replay premium to provide the relative win rates of Tier 1 minions would greatly appreciate it) 

Too easy to Triple

Low tier units are common choices for trying to Triple because of how relatively abundant they are and the fact that you can start collecting them from the start of the game. Traditionally, one of the main downsides of this approach is that you are saddled into the mid game fights with a pair of weak units while you wait for the payoff.

With sellemental you avoid this with its increased flexibility. Since the token goes to your hand rather than the board, you can bank part or all of the pair in your hand rather than having it clog your board. This lets you avoid the typical trade-off by letting you continue to scale your board into the mid game while retaining the upside a pair provides. Also, because you typically triple the token rather than the main body, you don't have to worry about other players contesting the pair because the main body will go back into the pool. Not to mention that, like other token units, the triple only costs 6 net gold rather than the full 9, or that by side-stepping board space it's much easier to "double triple" the main and token body...

This also lets you triple earlier and easier into cards like Nomi before opponents are strong enough to really punish you for it, shortening the delay to payoff per above.

Solutions

The above problems basically boil down into two themes: sellemental is too good with other Elementals, and as a stand-alone card it doesn't experience the same trade-offs that other early units do. I don't have a single suggestion what to do to change this, but some thoughts below for discussion:

Make it weaker

While it wouldn't do much for the Nomi hyper scaling, taking a bit off the stat line would do a lot to make it fairer in early fights - more similar to Alley Cat, where you get the economic benefit but probably take a bit of early damage to do it. 2/1 could work, or maybe make the token that pops out a 1/1. Also makes it easier to punish a greedy transition since you won't have as much Health buffer.

Move it to tier 2

Moving it up a tier would hit both sides of the equation: the reduced frequency slows nomi and makes it harder to Triple, and the economic impact is much more muted after the first turn. 3/3 sounds right; makes it more in line with the free money pirate and the steward of time. 

Make the token not an elemental

Would be a good change on the elemental scaling side, making it more like refreshing anomaly in terms of scaling power level. There is precedent for this type of flavor in cards like the Beast, not to mention more in constructed. Probably my favorite choice. 

Remove it

Maybe it just can't be salvaged. Replace it with some other Elemental normal type card like rockpool Hunter, or maybe if you're feeling bold a wrath Weaver type card to give Elementals a little bit better early scaling. If you were to do this I think you could afford to unnerf some of the other cards, like rag or djinni (but probably not both at the same time). 

TL;DR: sellemental, not nomi, is the reason for Elemental's unhealthy play style, in addition to being too strong of early an economy unit that enables high rolling and accelerates the game overall