Anyhow going down your points.. I find that the disdain towards tunnelers are contributed mostly by exageration of its effectiveness and downplay of ordinary sieges. Tunnelers arn't a fix to bad Pathfinding...
"+ Will make siege harder, because right now, a simple locked door can break most sieges ; add a moat and all siegers will be stymied."
-Disagree... Properly done Sieges can in fact be much more difficult to deal with then simple Tunnel Ambushes or even Tunnel attacking armies. When your Champion has to face down Siege weapons including Anti-infantry Siege weapons... You know.
"- Maybe a little to hard, because there will be no way to stop intruder to dig to your fortress."
-Goodness... Yeah... It isn't like there arn't walls in the game... and it isn't like you cant set up extra tunnels around your base to snag enemies and thus kill them before they enter... and it isn't like you can build traps inside your fortress or those tunnels... Yep none of those things are in Dwarf Fortress currently
(Sarcasm)
"+ More realistic : Many animals can mine (eg. ants, worms...) and invented creatures like antmen etc. should be able to do it too. And dwarves aren't the only creatures who mine, humans do it aswell."
-Though many creatures can dig, the length in which the fortress goes down to is usually much much greater then an ordinary animal. So Realism doesn't have too much to do with it.
"- But no animal dig dwarf-sized tunnel, and we can imagine that dwarven tunnels are very complex structures, inaccessibles to the engineering of other races."
-Perhaps small animals... Larger animals I can more then picture digging large tunnels. Though more likely uncommon you can close off areas or collapse them if you don't want to interfear with your design. Afterall, you can still build walls.
"+ Could be an interesting underground feature : An adventurer will be able to explore tunnels and find an underground nest."
-No complaint here
"- Cave do the same."
-I disagree... they both offer slightly different flavors especially since tunnels are intentional while caves are mined by nature. If you go down a tunnel you know it has to go somewhere while caves are just mindless (I hate them as they currently are... I welcome their improvement with open arms)
"+ Give a hint about the location of some features and minerals.
- And spoil the fun by the same way."
-No that is a plus for both... That is why people often build over caves.
"- Could worsen the pathfinding situation.
+ Or simplify it, because there won't be total blockeade anymore."
-Did this need to be said?
"- The monsters will be able to destroy the fortress design.. and break into a magma or water pool, spelling doom on the fortress.
+ The tunnelers AI could be made to avoid warm / damp wall, and walls could be reinforced to be inaccessibles to the tunellers (in the same fashion that walls are smoothed or engraved)."
-Assuming it was possible for them to do so... it would mean that the player would be challenged to build their fortress in such a way that it isn't sensative or "Instantly destroyed" by enemy ploys such as flooding. I will say though that unless there is a method to repair rivers or lakes I probably wouldn't want this in... Cool but it leads to other problems that will make extremely long lasting fortresses seem silly.
--On a side note: Destroy Fortress design? Build walls! but in all seriousness I think this is a hurdle we need to destroy as forcing the player to be fluid in their designs and to compensate for problems and advantages they find along the way should be part of the gameplay. The fact that people have become complacent that "Nothing happens the end" has become part of their Dwarf Fortress world is concerning.
"- A lot of people agree on the fact that ennemy tunnelers should be slower that dwarves at digging."
-Dwarves are so masterful at digging because it is the entirety of their race and something they have to do almost constantly. Other races do not. I think the Dwarven Advantage to Mining should come from the fact that Dwarves have herds and herds of amazing miners. I don't think we need to make everyone else horrible at digging.
"- Some people thinks that digging should be the last measure of an invading army."
-No... Especially since it is a much weaker tactic then proper sieges... Now if you could dig siege weapons into their base... MAYBE... that would be a start. However Digging ambushes, especially if your prepared for them, are rather minor. What do you do when enemies have 6 soldiers in your base? Kill them! What do you do when enemies parked six catapults outside your door? Kill them
--When I think about it... Tunnelers could be their harassment unit until you remove their above ground siege all together.