Thanks for the reply. I agree that it is difficult to say exactly what makes fun and what makes stressful - that can easily vary from group to group and person to person.
I personally think that the best approach is to always underscore that it is up to each individual community, and by extension up to each tournament organiser, what they want to do. But to at the same time have some sort of guiding principles or philosophy that you as the game designers recommend, should anyone be interested.
I personally think/agree that the most sound guiding principle is the recognisability principle. This is certainly what WM/H base their guidelines on. It boils down to that when an opponent looks at the collection of models that you are going to field in this game, ideally, they should be able to tell who each model represents without having to ask you. So you can convert and proxy all you like, as long as each model is uniquely itself in terms of visual identification.
This can unfortunately be trickier than it seems. One example is when two different fighters have the same weapons. Like Marcus Furious and Mago with their nets and tridents. If I make my own gladiator with a net and trident, how can my opponent tell whether it's meant to be Marcus or Mago? AR brings this issue to a point by having no limits on force construction, so both Marcus and Mago can be fielded with any combination of models in the game.
So it's all going to be about context. And there isn't really anything to do about that. The main thing that I think is interesting to discuss is the attitudes we each have on the issue. Cultures will always develop. On the GW forums I have read, people regularly build "counts-as" armies, where one race is brought in to fully substitute for another race. On the PP forum, there is generally a strong tendency for people to point out when conversions don't follow the approved guidelines. So very different cultures. And I'm interested in the Arena Rex culture, if there is such a thing yet.