>Thieves are able to hide on roads and Rangers are not.

Balance. Few people would play a thief if they were that limited to cities. Give them uber powers you say? I'm not sure I'd want to see thieves be much more powerful than they already are.

>I'd also like to see Invisibility detects become harder to
>obtain.

All the ones I can think of off the top of my head have drawbacks, and all require you to step out of the shadows. Most require you to walk thorugh wilderness areas. Set a snare and it's bye bye thiefy. Seriously. Imagining how a sufficiently bored person might play a ranger, it makes me seriously ponder whether to ever rank a thief into snare range.