I prefer not to have fraud.

Ideally, prices should be consistent with effective costs.

Practically, I would be content if prices of any single leg couldn't be negative. If leg A costs a and leg B costs b, the cost of A+B can be lower than a+b (you can offer bundles, discounts, etc.); just not less than either a or b.

If you play "tricks", the consumer has the right to use such tricks to their benefit.

Lower profits for airlines.

I think ticket prices should reflect a combination of the cost of goods sold with a reasonable profit margin, supply and demand, and competitive pressure from market forces, with regulations to prevent fraud and anti-competitive behavior.