So I'm way late on this discussion but I was reading through the thread and thinking about this today.
I always have a tough time comparing fighters from different eras. To SHAKYR's point, there have been some advancements over the last 100 years in terms of what fighters have at their disposal. I do agree with you that much of the training has been consistent, which is actually a credit to the sport IMO that some of these techniques have lasted the test of time.
That being said, if you literally go back 100 years, one of the most significant differences is the overall health and life expectancy of people. In 1914 the average American male only lived to be 52. Today we've got a guy nearly that age who is still relevant at light-heavy.
Then of course you look at size where Jack Johnson was called the Galveston Giant at 6'2, which would put him at a considerable disadvantage today against the biggest guys in that division.
If you gave some of the past fighters the same nutrition, training, and equipment that fighters have today, would some of them be able to compete? Absolutely. And I do think fighters at that time maybe in general were a little tougher, but you also have to consider that the life of the average person was really difficult, so the average person was a bit tougher as well.
The further into the 20th century you go, the easier the argument is to make IMO that some of the greats could compete with today's fighters. I think guys like Robinson and Ali would still have made an impact today. When you start talking about guys like Ketchel it's a lot tougher to know for sure.
Lastly, I'll say that one thing you mentioned did strike me as a little odd. You mentioned having less divisions but that you'd want fighters to weigh in on the same day. I think you can make an argument for both of these things separately, but together I don't know if they make sense. Because people today are better nourished than they were 100 years ago, you've got a larger spectrum in terms of heights and weights. The idea of having fighters weigh in on the same day as the fight makes a lot of sense IMO. You'd have more people fighting closer to their natural weights and take some of the danger out of the cuts.Still, if you're going to do that I think you've got to have enough classes that someone can realistically make a weigh that's somewhere close to what they'd be naturally.
On the other side of that coin I can understand the interest of combining classes. You'll have a better talent pool at each weight and can likely make better fights and draw more interest to the sport. But if you're going to do that you'll force fighters who naturally way well above or below the limit to be at a disadvantage. So in that case I think it makes sense to give someone more time before a fight to recover.
Historically you'd see guys like the aforementioned Ketchel or Joe Gans move up in weight to fight Heavys like Sam Langford. Was it probably entertaining? I think so. Did the smaller guys typically win? Not too often. So while it could be interesting, I'm a bigger fan of the second idea of same day weigh-ins. I don't know if that and the other idea are compatible.