My little advice (maybe it can help):
If you are getting the game too easy, a good method to keep the challenge is to stay in low tier equipment. Using tier 3 with level 1-2 material for example.
Also, i figured out that if you only keep doing main quest for region, and closing rift, there is a path that you can follow on nightmare and keep the challenge:
- hinterlands, in the meantime travel to val royeaux to unlock the next main mission
- By this point you will be lvl 7-8 wich are the recomended for doing the next main mission (almost every main mission will be like this, no more than 1 or 2 level higger than recomended).
- storm coast
- hidden oasis, till you open up the temple.
- once u get level 11, do the next main quest, i will be pretty damn hard at this point.
- Here come up to you, i prefer always to go to the map where i find the needed items for my especialization: Western approach, Crestwood or Fallow mire. Once you "complete" one move to the next (complete just main quest and close rifts).
- If you are still under lvl 15, go and start doing exalted plains or emerald graves. as soon as you get lvl 15 leave the area
- do the next 2 missions, the order doesn`t matter, so do first what you like more.
- At this point you will be lvl 16, or maybe 17, so, go back to emerald graves and exalted plains and "complete" what you left behind.
- move to hissing wastes or Emprise du lion. Here you will get tier 3 recipes (actually i get tier 3 armor recipes at lvl 11, as soon as i unlock hissing wastes, just because i like a lot the desing of it, you can do this also, and use lvl 1 materials to keep the challenge of the game).
- Do the final act of the game once you reach 19-20. Actually, even if you do it as low as 19, it would not make any challenge, since the final part of the game is not harder than any dragon fight, maybe its even easier (i fell like it is), it will be even easier if you are over-geared, so, by doing it with crafting equipment with lvl 1-2 materials can make a challenge, or even lvl 10-14 unique items.
Take a look on a wiki, to also farm all the runces schematics, because if you miss one, the palces where the higger tier runes schematics are located will be low level (you need all the runes schematics to get higger level ones)
Also, if you want to do all the 10 dragons, its a good idea to do them no more than 2 levels higger. It will screw up your level proggresion of course, since dragon gives a lot of xp, however there are some editor on the net that will let you edit your level and skill point, so, you can do all the dragons and then edit the char, removing the xp and (if level up) skill point that they gives. (EDIT: Forget to metion, in order to remove skill point, you need to use a respec. amulet first, if you already used that skill point, otherwise, even if you reduce the lvl, the skill point will be still there).
If you like vivienne, or knight enchanter spec., take it without the pasive that restore 30% of the damage done as barrier, good luck getting into melee without that pasive on nightmare, undergeared. Since patch 3 Spirit Blade is no longer an a eldritch detonator ability, so it can be used without risk of becoming OP (It was doing cross-class combo in on same ability, since spirt damage can sleep, wich is rogue debuff for setup combo)
Also, there are some people that are runing solo nightmare games, other by doing duo nightmare, for those clases that can`t (yet) solo the game. At the end, is up to oneself to keep it fun and hard.
About if we get overleveled, yes, thats is right, but based on the fact that we can still keep playing the game once the final mission ends, i believe that there will be a lot of story dlc post-final mission, maybe with new locations for levels 20-25 or even 20-30, maybe that is the reason why we can get level 23-25 right now.