I wasn't going to reply to this since the game comes out in a few days, but hey, let's give it a shot. The only thing I wanted to say was that I don't think electric terrain (I'm not going to comment about the other terrains/tapus), with tapu koko, will be inherently OP like some people are saying. Yes, you can throw out calcs of tapu koko breaking walls such as chansey, but that's exactly what electric type needs. It currently has no "effective" physical attacking wall breaker, leaving it very vulnerable to common special defensive mons. In my eyes, this is one of the major contributing factors to why electric is a lower-tiered type, and why tapu koko shouldn't be a "quick ban".
Additionally, let's look at electric terrain itself. 50% boost to electric type attacks, only affects grounded mons. This means any pokemon with levitate, or flying type, won't receive the bonus. Out of the majority of electric mons commonly found on teams, half of them won't even receive the terrain boost. This means the zapdos/rotom core won't receive a boost (being walls, that's fine), but other common attackers such as thundurus(-t) and eelektross also won't be receiving any benefits from the terrain. However, other common electric mons such as both megas, magnezone, raikou, tapu koko, and alola-raichu will reap the benefits. Even further on top of this, the only pokemon with synergy from its ability is alola-raichu. Lastly, it prevents sleep which is nice for the annoying breloom spore tactics, the odd smeargle, and it also prevents rest stall, but nothing major to write home about. Just for reference sake as well, electric type hits 2 types super-effectively, 3 types not-very-effectively, 1 immunity, and the remaining 12 types neutrally.
If we compare this to other similar effects in the game, i.e. rain, I don't see why this is considered ban worthy, while those are not. Rain gives a 50% boost to water moves, reduces the damage of fire moves by 50%, and has synergy with swift swim and hydration (and rain dish/dry skin, lel). Water types also have the luxury of pokemon options when building a team, giving them access to effective walls and wall-breakers (both physical AND special), who have good type coverage attacks. For comparison's sake, water type hits 3 types super-effectively, 3 types not-very-effectively, and 12 types neutrally (overall +2 over electric being 1 more super-effectiveness and no immunities). The major difference here is the activator mon of the weather effect. Tapu koko is a great mon all around, albeit has mediocre bulk, but great speed and attack/spa. Politoed on the other hand is almost a "wasted slot", being that it's there to set rain, but doesn't that contribute much to the team otherwise. This is easily balanced by the wide array of great mons that water type has access to. Kingdra, m-swampert, azumarill, manaphy, keldeo, m-sharpedo, the list goes on. That being said, when tapu koko is released, it will become a staple on electric teams. There's no reason not to add it, and you don't have to build a team around it for it to work. But, if you are building a team with politoed, you're building a team for rain. Basically what I'm trying to say is, we already have weather meta in the game, and you don't see people quick-banning drizzle or drought or sandstorm (or snow warning, lel). Obviously electric terrain on tapu koko will be strong, but I don't think it will be as strong as most people say it will. Sure, it will likely boost electric type usage, but I highly doubt it'll become top tier and unhealthy to the meta.
There's probably more that I could say about type coverage, options of strong/useful stab moves, type coverage for electric mons, etc, but it's late.
Edit: I don't think the addition to these terrains is comparable to weather wars in gen5. Back then, weather was permanent, meaning there was no way around it unless you had a weather user yourself. In ORAS, weather was no longer permanent, lasting only 5 turns (with the rocks being banned), and it isn't a weather-centric meta. These terrains are also only lasting 5 turns, not permanent. That being said, there's no reason for these terrains to be inherently stronger than the current weather meta (aside from specific type-boosting advantages such as psychic terrain).