Every time after I've built or had an aviary
built for me I ask myself this question and I'm always tweaking something for the next one.
A friend of mine who has over 50 aviaries was having rodent problems and his biggest regret was that he hadn't used the wire with the thinner spacing, which he was now slowly replacing the old wire on his aviaries with at great expense.
I thought I'd take a lesson from him here and have some breeding flights built with the wire with the smaller spacing instead.
I also laid down solid concrete floors so that the flights are completely rodent proof.
The problem is I'm not really sure it was the right thing to do because:
a) it makes it more difficult to view the birds through the thinner spacing
b) it will make it more difficult to sell the flights should I ever decide to because the thinner
spaced wire isn't really suitable for parrots so I'll be restricted to selling it to finch keepers.
c) I'm not sure how needed this was anyway as my dachshund is a better ratter than 10 cats and can sniff out a rat or mouse from a mile away!
So even when you think you're improving on previous designs, you may find that there's a trade-off!
I have a community display aviary next to the flights and this has the regular sized spacing and a soil floor which is planted with shrubs - it is much nicer to sit and watch the going-ons in this aviary, not just because it holds more birds but because they're so much easier to view and it looks much more natural without the concrete floor (even though the flights do have plants in pots inside).
That's maybe another thing I would have done differently - more mixed communal aviaries
rather than the single species, single pair flights I have now.
It would have meant less pairs of twinspots and cordon bleus (which have always been my favourite and I've housed them in individual pairs in the hopes of having success with them).
Individual pairs = more flights, more costs + more work feeding and watering.
I think that perhaps if I was to start again all over from scratch I'd go the route of a few very large communal aviaries instead and enjoy birds interacting together in a natural setting while having less work to do at the same time

There may be some however (like my friend with the rodent problem) that think I've gone about it the right way.
Whatever the case, I think my aviaries are an asset to me and perhaps the most important thing is to look after them well so that they can bring me many more years of joy and wonder!