Wanna dispel a common misconception that building with cores based purely on typing is an effective way to build. This method is archaic in such an inflated meta. If you want to build an effective team, with a defensive backbone, it's much more effective to cover certain threats in the meta. Take for example, off the top of my head, Jirachi+Rotom+Latios+Lando+Keldeo+Mega; this team has few glaring weaknesses and is representative of the majority of bulky offence builds in some form. It doesn't feature any of the fabled FWG or SFD of DPF cores or whatever you want, yet it isn't auto-losing to a hell of a lot of the meta, why is this? It's because role compression and blanket checks are a hell of a lot more important than typing cores now, and always will be in an inflated meta.
Another massive mistake I see a lot in the OU room, and on here, is people arbitrarily asking for Pokemon to fill X role. For example, questions such as "What's a good special tank?". This is not only ineffective, counter-intuitive, but once again, outdated. There are a multitude of Pokemon which excel at taking hits on the special side, but it's pointless throwing on a Chansey, though it may be the premier special tank/wall, onto a bulky offence/balance/semi-stall team if you're struggling with Keldeo (yes I know Chansey doesn't beat Keldeo this is just an example), Volcanion, Zard-Y, etc. Identifying key weaknesses and throwing on something to remedy these is much more effective. This is thanks to the inflated meta, once again, and the fact that role compression and blanket checks are so so important, so important that I cannot add enough emphasis. The key to building a solid team is recognising what Pokemon provide support to the other members, and recognising your team's weaknesses and remedying them without using arbitrary Pokemon because of some made-up criteria that you must fill (see: typing cores, "what's a good tank?" "what's a good sweeper?" etc.). Letting go of this will make you a much more efficient builder, I can promise you. The lesson of these two paragraphs is: don't throw on random typing cores or Pokemon to fulfil pointless roles on certain teams. This is a hindrance to building now, and will get you nowhere outside of serendipitous fringe cases where the team will work. I hope this makes sense to people, because my building and stance on it is convoluted to explain, yet second-nature to me now.
As far as Megas go, I don't even recognise the term. Other than the one-per-team restriction, all they are is additional Pokemon, in essence. What I will say though, is that considering many people use a Mega as a starting point whilst building, and what they bring to the team in terms of role-compression, offensive prowess, or utility, it is unlikely that, a) you will find a team without a Mega, and, b) you should not be using one yourself. Their effect on building is this: you are at a disadvantage by not using one, and a lot of them are detrimental to the meta in conjunction with the many other viable and threatening Pokemon we have.
As far as hazard control goes, many teams are forgoing it as of late, which is acceptable if the team doesn't autolose to hazards stack or have multiple Pokemon weak to Stealth Rock, or a Pokemon such as Talonflame, Zard-Y, or Volcarona. As long as you can either provide enough offensive pressure to common hazard setters, or you team doesn't mind hazards too much (see: the random 5 'Mons I posted earlier), you're ok to drop hazard control in favour of coverage/Specs on Latios or Roost on Latias or just a different Pokemon altogether. Not going to say too much about how I feel about hazards in general, though, but I will say that I do not like the concept at all.
tldr: don't build in a shitty old way, role compression is king, you should have a mega, hazard control necessity is a team-by-team thing
edit: @below I'm unsure as to what you want in response, if I'm honest. They are incredibly powerful additions to the game which provide a hell of a lot to a team, and if you want evidence that teams are less viable without a Mega then go and watch all of the last OR/AS OU WCOP replays. That should be evidence enough that you should be using a mega, and that they greatly increase the viability of a team. Teams are unique, and have unique needs, as was the message of my post.
People who come to these threads come here for two reasons; to learn about battling and the current state of the meta, and to discuss battling and the current state of the meta. If they fit into the former category, they shouldn't be innovating anyway, for the most part, as innovation from those who are still learning the ropes ends up in disastrous sets like Specs Scizor or whatever these magicians cook up. The latter already know what is viable and have a grasp on what has merit as far as innovation goes and what is just a gimmick, so they're completely unaffected. I'm not saying leave innovation to the best of the best, just learn the meta before you try to innovate. After all, an innovative set is one that takes advantage of common meta trends, and if you don't know said meta trends/the meta, then you aren't innovating.