1. Doping allegations documented by BBC Panorama into Nike Oregon Project coach Alberto Salazar, who has denied any wrongdoing.
"Most track and field athletes have heard these allegations before. If they prove true, it's great that a powerful journalistic entity like the BBC can provide such a safe haven for people to bring forward information or evidence.
"My biggest fear is when sponsors are effectively in charge of the whole sport. People are scared of saying anything negative which might reflect poorly on their sponsor because they want to get into future races and negotiate better contracts.
"Regardless of whether these allegations are true, there is a push to take the biggest scientific advantages possible. They might be 'grey area things' which aren't illegal, but offer potential incremental advances.
"It's not necessary to fulfil your potential through expensive and complex methods. It's also debilitating to those who might think they'll never have access to those types of resources, so what's the point in trying?
"Most stuff is for the placebo effect and people are thinking far beyond what is necessary. I've done this over a decade and reached times I didn't think were possible, simply by eating what your grandma would put on your plate.
"The win-at-all-costs mentality says you don't care how talented anyone else is. Our goal should be to reach our own individual level. Often you'll be beaten by someone better. Maybe you should just congratulate that athlete and be satisfied with your best."
2. Allegations of age manipulation, particularly in relation to African track athletes which feature, for example, in the book Foul Play: The Dark Arts Of Cheating In Sport.
"It's not for sinister reasons, it's so they can get out of their country and get a visa stamp deal.
"The general theory you hear from many Westerners, agents and coaches is that it is difficult for athletes from a Third World country to get a visa in their passport to race overseas. If you're a talented athlete, you want a chance to earn money - it's your right. We take it for granted in the Western world.
"If you qualify for a major event like a world championship, the host nation must guarantee visas for athletes who qualify. For first-time athletes, it's difficult for them to make their teams, like in Kenya or Ethiopia, who possess some of the world's best athletes. World youth or junior teams are easier for them to make, so they will adopt the passport of a sibling or manipulate the papers - if they had papers in the first place. This is allegedly the way it happens, so they get the right to travel.
"Once they return home, they're proven to be a safe, legitimate traveller who will not suddenly become a vagabond in a foreign nation. From that point, they can get visas to race accordingly. At this stage in my career, it makes no difference whether they're 16 or 36, it's just a case of who is the best guy out there."
3. Going to a longer distance like the 5000m in future campaigns.
"I've dabbled in it. The Commonwealth Games didn't go as I had hoped [Willis finished 10th at Glasgow] but I had a good result a month ago in New York [finishing second in the Diamond League meet]. There's a temptation to give it a serious crack, but I have unfinished business in the 1500m at the world championships, and most likely Rio next year.
"I don't see any signs of decay in my body. If I'm able to go until the Tokyo Olympics, it will give me a four-year period to give it a go."
4. The future of New Zealand track and field.
"There's a lot of talent in a variety of disciplines, apart from sprinting. The initiative of Rio 2016 which Athletics New Zealand put forward some years ago has gone well. I laughed it off initially, but they've proven me wrong. I generally thought just throwing money at something won't work; you need to have coaching and so forth. But give kids opportunities and they are willing to make sacrifices.
"The biggest challenge in the Western world is [retaining athletes] between the ages of 18 and 22. They have to ask: Is it worth putting in the hard yards when they're potentially sacrificing a fun social life and advances in their professional futures? I think kids are starting to realise it's worth the effort. They get to see the world and enjoy some unique experiences. There's no reason they can't finish their studies later."