Mod tools aren't the only thing.
They also need to be accessible.
I'd really like to make some maps for Star Wars Battlefrontt I, but the modding tools package is the worst I have ever seen. It is clearly made for already experienced users, which greatly decreases the number of potential modders.
Of course one could say, the more complex the tools, the less inexperienced newbies will use them, which would - from that perspective on - lead to a better quality.
But in my opinion I doubt that this will be the case in all of the times.
Because great modding skills also need a lot of technological knowledge, and on the other hand I strongly suspect that a "techie" bears at the same time the needed storytelling skills, because storytelling skills are so much different from technology skills, that they are almost - imho - incompatible to one another.
Which makes me fear that people with great modding skills are able to create mods that are just great in a technological sense, but which are at the same totally lacking any storytelling qualities.
Of course, nowadays, storytelling quality might even not be wanted at all - especially by the Action-RPG generation, to which storytelling appears to be nothing but a hindrance, a distraction from gameplay. Blizzard's Action-RPGs, for example, have almost no story at all (and I mean the games, not any baclkground information presented in novels or so).
Following this thought, I even fear that the masses of people who believe that RPG = Action will bad-mouth pearls like PS:T, because of their great story-telling quality ! "Too much text", they will say, "it just distracts from playing !" Whereas "playing" would be = "hacking/slashing", in that perspective of an Action-RPG player.
Which leads - the more game companies - and especially developers - concentrate on the mass market for development costs revenue - to games with less and less actual stories/storytelling in them, which will be praised by the mass market who doesn't want "too much text" at all.
Times for lovers of stories look bad.
And Action-RPG lovers will surely say in an aggressive voice: "Go buy yourself a book ! If you want to have stories !
'Stories' are an abberant abomination nowadays ! They have nothing to do in RPGs at all !"