First of all, let me say that many of the things that bug me are more personal preferences than anything and may not reflect the thoughts of others. But I'll try to list some things anyway.
In regards to OpenGFX compatibility, the main problem is with bridges and level crossings (mostly the former), since these are based off of TTD graphics which don't match the OpenGFX ones. Obviously these things can be fixed by using the right bridge grf and different rail sets, but for those who don't, they will look broken. Of course, the ground tile mismatch is also a (minor) issue.
Now for a few of those quirks, in order of importance to me:
- Curves on the "country" roads are drawn with just a single line instead of being double yellow like they should be, whereas the "city" roads do it correctly.
- There's a similar inconsistency with the slopes; only the front facing ones include a passing stripe along with the solid one, while the back ones show just a single line.
- The straight country road pieces have a slightly irregular spacing in the stripes. The front facing slopes also have an odd passing stripe that doesn't match the rest of the stripes (they're too small).
- The "city" curves don't show a sidewalk like the straight pieces do.
That's all I can think of at the moment. Those last two are very minor, and I'm sure 99.9% of people wouldn't even notice. Overall NARoads is a great looking set, but if those issues I pointed out above were to be fixed it would be even better.
And just so I know, is the work you're doing a collaboration with OzTrans and company to improve the existing set, or will this be something different? Either way, I'm looking forward to seeing what happens