The RPG experience is videogames is markedly different from what it is in tabletop. First, and foremost, videogames cannot yet deliver on the sorts of experiences that tabletop RPGs allow simply due to differences in medium. That is a matter of technology and overcoming it may prove to be more trouble than its worth. Second, and almost as important, is that the commercial demands for a viable business imposes constraints above and beyond that which technology alone achieves. These two factors account for why the RPG in videogames routinely conforms to a themepark style of gameplay experience.
Expanding on a successful themepark RPG has risks, same as any other franchise expansion. Be it a MMORPG (such as World of Warcraft), or a single-player RPG (such as Dragon Age or Path of Exile), or some form of blend (e.g. Dragon's Dogma), you run the risk of souring the entire franchise when you add something that is dissonant in tone to the existing corpus. That's why you test early, often, and repeatedly well before you go live- and you start by making it work on paper, as if it were a tabletop RPG.
So, let's show where you start and that's with writing it down. I'll demonstrate using a fictional (but popular) example: adding Vrykul as a playable character race option to World of Warcraft.
(Note: This version is the version I envisioned during the time of the Cataclysm expansion and doesn't take into account what came up since. Upon request, I shall post a new one that I came up with to account for what we know now as of Legion.)
Vrykul Player-Character Proposal
Design Intention: Vrykul characters entering the ranks of the Alliance or the Horde are meant to be few in number and grow slowly in population over time. This is due to being assimilated by conquest in the wake of the Northrend Campaign against the Scourge and the Lich King. The requirements for creating a Vrykul character should be lightened or removed if future expansions create circumstances that permit it without creating an inconsistency with the lore to date.
Requirements: A player must have on his account the following to enable Vrykul as a character option:
- A character at the level cap for the desired faction.
- A character that is Revered with either the Valiance Expedition or the Hand of Vengeance for the desired faction.
- A character who has the Fall of the Lich King Achievement, either 10 or 25-Man.
Classes: A Vrykul may be a Warrior, Hunter, Mage, Shaman, or Priest.
Racial Abilities: All Vrykul have Titan's Legacy (Active, 3min CD; as Dwarf Stoneform, turns the player into an iron-skilled model for the duration). Horde Vrykul have Val'kyr's Kiss; Alliance Vrykul have Titan's Will (as Way of the Forsaken, same CD). They speak Common (Alliance) or Orcish (Horde) and their own language (Vrykul). Racial mounts are protodrakes (flying) and dire bears (ground).
Starting Zone and Play Experience: Vrykul characters, like Death Knights, begin above Level 1 and aligned with the Scourge. They have their own starting zone and play experience. Vrykul begin in Howling Fjord, fighting the Alliance and the Horde for the Scourge and the Lich King. The emphasis here is in proving one's worth, as the Val'kyr are watching on the Lich King's behalf, so that one can ascend in power and prestige to become one of the Lich King's champions. However, the war goes against the Vrykul and they conquered tribe by tribe and zone by zone to the feet of Icecrown until they are faced with an unbeatable force and faced with the choice of surrender or death. This choice masks the player's decision of which faction to align with, and the epilogue integrates the Vrykul character into the Cataclysm events with a survey of major changes post-Icecrown before feeding them to either Mount Hyjal or Vash'jir.
Part One of the zone is where the character spawns. He is Level 75, and he spawns just outside Utgarde Keep. His initial mission is to repel the Alliance forces besieging the village right outside, followed by providing relief to the flanks held by the Horde on the cliffs to either side. Then he is sent away to assist the Vrykul at Skorn and at Gjallerhorn, only to find that in his absence Utgarde Keep has fallen and King Ymiron had been slain when Utgarde Pinnacle had been sacked. He fights to protect a retreat to Grizzly Hills, only to fall under a two-pronged attack by the Horde there; held in place by the Orcs while the Forsaken bombard them with the an early version of the Plague. Forced to flee, he gets the call to gather at the Wrathgate, where he participates at the side of the Lich King until the Forsaken plague ends the battle.
Part One introduces the character to Sylvanas and Tirion, and through Tirion hears about Varian. Sylvanas and Tirion take note of the player late in Part One and begin to order their subordinates to target the player in particular, recognizing the player as a rising Scourge champion. Basic abilities for the class the player chose are parceled out similar to how Death Knight abilities are, and specific quests are designed to make use of them; this, by necessity, means some quest tweaking to ensure the success of this goal. Talents are likewise parceled out slowly, instead of dumped all at once, and quest design adjusted accordingly. The player ends Part One at Level 77.
Part Two begins with the player in Icecrown. He is sent to assist Scourge forces in resisting the Argent Crusade's advance into Icecrown, and then in resisting the Ebon Blade's attempt to establish a base in Icecrown, before he is sent to lead a large Scourge force against the site of the Argent Tournament. When that raid is defeated, he fights in the Arena against Sylvanas, then against Varian, before he faces Tirion in a fight that the player cannot win; beaten, he is sent back to Icecrown to tell the Lich King that the Crusade marches on Icecrown. The Lich King orders the player to stand at the foot of Icecrown and hold the line at all costs. This battle, against the Crusade and the Alliance, goes just as badly as expected; overrun, his forces destroyed or fleeing, Tirion and Varian confront the player- surrender or die. If the player chooses to surrender, his weapons are broken and his armor stripped from him as he's taken in chains away. If the player chooses to die, Varian executes him; Sylvanas appears with a Val'kyr to raise up the player as undead and takes him away. Part Two ends at Level 80.
Part Three is the Epilogue. The player's character now has all Talents and abilities of any other character of that class, and receives from this series of quests a set of gear one would expect of any character who had moderate success in raiding Icecrown Citadel during Wrath of the Lich King, enough so that he is not immediately replacing gear upon his first steps into Hyjal or Vashj'ir. It ends with Varian or Sylvanas formally accepting the Vrykul into the Horde or Alliance; the character's Reputation is Friendly with all other races and Honored with his own as well as with that of their patron- a reflection of the prestige of that leader. Then he gets the breadcrumb to start on Cataclysm and goes from there normally.
Notice that I rarely talk numbers here, and then only as easy references to known elements (racials) or gameplay benchmarks (levels). The idea at this stage is to see if the concept itself conforms to established canon of lore, has a compelling concept that compels further elaboration, and otherwise gets the attention of the rest of the team. Using a known example (Death Knights) helps to make this easier to pitch successfully, and allows discussion to move immediately to details rather than spend time hammering out basic expectations. It also makes it easier for you to avoid making something that won't sit well with what your player expect out of your game.
What would a World of Warcraft player expect out of a Vrykul addition? Given the time I mention above, it would be utterly dissonant to NOT play out a starting experience like what I outline above. There is no other easily-accepted way to make playable Vrykul happen successfully at that time. (That is not the case now. Legion did a lot to change the circumstances.) There are other ways, but they require far more work to achieve the same end; it involves making use of all the lore surrounding Ulduar, but the structure is the same: fight Alliance & Horde, lose, choose to go with one of the winners. The reason it's harder? For all the love of Ulduar as a raid, the dungeons that went with it weren't so well received (especially Halls of Stone), and the excitement of playing the losing side of the entire reason for the Northrend War is lost- only in retrospect would anyone involved realistically perceive the important of all of the events involving Ulduar. Far less risky to go with the dramatic hotness of the Lich King.
There are other risks involved, but you eliminate most of them from the get-go if you take your property seriously when you aim to expand upon it and fulfill the expectations that you'd already established for your audience- expectations that your audience took up and invested greatly into. Don't fuck that up; give them what they paid for, literally and otherwise. Don't include anything else- anything at all. They want a classic cheeseburger. Don't give them tofu patties.