I know this must sound like a odd question, and probably an even odder conclusion for our beloved Warden's from Dragon Age Origins, but hear me out. Perhaps in game wise, as another explanation besides external reasons why the Warden can't or doesn't resurface in these continued universes is because they are just to powerful in relation to the people in the rest of the world. Crazy right? Well lets look at it from a in game perspective for a moment.
In Dragon Age Origins Warden if obviously the main protagonist so let's examine exactly what the Warden can potentially do on a personal level. To do this we have to look at all 3 classes, their abilities and specializations.
Mage Warden, probably the best example of just how powerful the Warden could be is right here. It is no secret that the Mage was one of the best things right out the gate in Origins, high damage, versatility, fully customizable to handle various roles within the party, from Damager, to Healer, to Controller, to Tank, the Mage could do almost every job. From a in game perspective Mages are feared, and in this case its probably justifiable, The Mage Warden is praised by Irving as his prized pupil, who even finishes their Harrowing in record time according to Cullen, Jowan during the Origin also praised you as a Great Mage as being smart and deducing you could probably avoid the Templars had you the desire to run away from the tower despite you being his junior, You are a badass right from the start, and depending on your choices in the origin, you can show yourself to be a no compromising straight laced staunch upholder of Law and Order keep true to the Circle, or a Loosey Goosey willing to bend or outright break the Rules the end result is the same regardless, you become a Warden. Let's take a look at some of the examples as a Mage that make you particularly noteworthy.
During Nature of the Beast you can happen upon the Arcane Warrior specialization, a unique blend of Mage and Warrior that was until that point lost to time, the Mage warden can learn and utilize this specialization at a later point which in turn makes them the last Arcane Warrior in existence that we know of. If you go by the hyperbole and abilities shown in game the Arcane Warrior is indeed a very very powerful specialization that could easily make it seem the Warden themselves are Superhuman, (if that could even apply to someone who can already cast magic). Still this knowledge was given to the Warden so its fair to say that if you teach it to anyone else in your party it's because your Warden taught it to them, on the same note you could choose not to teach any of them and that makes the conclusion of the Warden being overpowered seem that much more likely.
Shape Shifter, Bloodmage, and Spirit Healer are used by other people throughout the lore so their really is no claim to be made about a Warden who specializes in those abilities specifically, but one could argue that if Morrigan and Flemeth themselves are powerful in part because they are Shapeshifters then it might also apply to the Warden (Personally when I got DAO pc i grabbed the enhanced Shapeshifting Mod quickly and never looked back, it corrected the Specializations glaring problem and added more variety.)
In Awakening, two more Specializations are added, Keeper and Battlemage, while the former again has been seen partially used in other games, and by its name is likely widely known throughout the Dalish Clans, the later we have not really seen since Awakening, and it is powerful. Draining Aura, Elemental Chaos, Stoic, Hand of Winter, combined with spells and passives like elemental mastery, attunement and fade shield, it's clear to see the Mage Warden would be nearly unstoppable in some cases, also honestly we've seen the scope of most mages in origins and just how powerful a first enchanter or senior mage is, and the Warden surpasses all of them by the end of Awakening, not to mention Amgarrak and Witch Hunt..
If a Warrior or Rogue the praise and proven ability of the Warden is no less phenomenal, a City Elf Warden can nearly single handedly slaughter the entire palace of the Arl of Denerim including the Arl's son and his two lackey's with nothing more than a borrowed sword and whatever gear they find along the way.
A Dwarf Noble can enter their own proving and defeat some of the finest warriors in Orzammar, and survive long enough in the Deep Roads with no armor other than what they find and what weapons they are given to join the Grey Wardens. Your father either calls you the best fighter, or quickest fighter in your house when he speaks of you to Duncan. You can kill Lord Dace's song in a honor proving if you choose to press the issue between the two of you, and Mandar is supposedly a accomplished duelist himself.
Dwarf Commoner is pretty much feared as one of Beraht skullcrackers, where in its pretty clear that you are the better fighter between you and Leske, you also go on to win a proving despite fighting veteran warriors, and if you ask Leske about whether or not you took some of the guards down with you upon capture he remarks you did indeed, besting several of them before they finally captured you. You then with the aid of Leske kill everyone in the old Carta including Beraht.
Dalish Elf is probably the most tame in terms of accomplishments but you are recognized as a hunter or at least are capable of hunting according to Duncan's introduction of the Dalish Origin, where he comments your quarry isn't always the local fauna.
This is already going on longer than i intended so in a effort to short it fill in all the accomplishments, levels gained, skills potentially learned, weapons obtained, armor obtained, either of which could be created, and just compare it all to that of which Hawke can do, and judge for yourself. Now with DA:I nearing release, and from what we've seen, while it may not be a competition, i can't help but wonder if they are capable of reaching the levels of sheer levels of epic