I like everything said here except the mentality that the respawns should be super short. Even now I was out to get the NM hunter achievement and boy was I dissapointed. All NMs "big" and small are on a timer (most boring pop mechanic ever that should be one of many different pop conditions, not every NM), and a suuuuuuper short timer at that. These mobs are not rare in any way and that completely takes away the whole point of NMs for me.
For me NMs should mean:
Some epic fights (NMs right now don't give that, not all have to I mean if they are lower level I can always pwn with a higher job/class, but HNM... they should for sure, and even some other NMs should)
Good drops, not all NMs of course, but atm there are basically none (I think XI had a good balance there).
AND they should be rare to see. Should there be some NMs that has quite a low timer? Yes of course, but at the same time the majority of them should be on a timer that at least doesn't have the NM up at all times as it is now. HNM should be RARE, it doesn't have to take up to like 12days to pop a King Behemoth, but it should still be rare and 6h spawn is not rare... sure a regular Behemoth might be around there (I would like more considering it is a freaking behemoth... at least 9h, but the King? No way). The window and respawns should definitely be long enough to make NMs a rare sight, so that if you happen to run past and see one you go OH COOL.
Lottery spawns should definitely be there for regular NMs too, the more different spawnconditions the better, there are several good ideas in this thread.
Now, I am not saying they have to bring back XI timers on NMs/HNMs, but they do have to make them a lot rarer than they are now. Fighting epic mobs can be done in all the instances, forced spawn mobs, "KSNM/BCNM" type battles. These can be fought at any time. HNMs especially, but NMs too however should be those rare exciting battles you don't get to do or even see 10 times a day imo otherwise you might as well make them forced pops and instanced like everything else.