[color:"orange"]The game needs to "remember" the mana regeneration rate for the different walkpoints the player passes through.[/color]
The game would have to change music and environmental sounds (etc) when changing areas anyway, so changing regeneration rate should not be too hard, as long as it is constant within each major area. If there are going to be variations (a cursed building with a lower regeneration than the surrounding forest, or an oasis with higher rate than the desert) perhaps those areas could be designed as a mini-level.
Whether you would want to vary the regeneration rate (effecting specific character builds much more than others) or if this feature would be the best use of resources during development are other matters entirely.
[color:"orange"]Unless it's limited to only certain key areas, I think it's better laid off to rest.[/color]
It would probably be limited to key areas anyway. You could get really detailed and have the recovery rate differ if (for example) your character was walking on a stone pathway rather than the nearby grass, but practically only larger difference would be noticed by most people. Even then, potions could be the main source of mana (at least during battles, and depending on price / availability), with people not paying a lot of attention to their mana levels while walking around exploring.
This could be an interesting difference between classes or races, but in that case you would have to make sure there were areas that each character type would find easier or harder, or there would need to be a party, so individual strengths and weaknesses could be balanced. This could more easily be applied to opponents (ie provoke a magical swamp monster into leaving the water and it could be much easier to defeat), though it wouldn't necessarily be the best way to differentiate them or modify their behaviour.
