There's a lot to be said about the power of limiting choice. It's very popular to give the players as much choice as you can. That's good in some ways, but bad in some ways. The more choice players have, the less control over the flow of the game the developer has. Especially in persistent world games, this can lead to player malaise and a general feeling of not going anywhere.
There are two popular methods of giving better control: 1) Restricted actions, unrestricted space. This gives the illusion of unlimited choice without any of the actual ramifications. I hate this solution because it produces painfully bland play with zero agency. 2) Quests. Basically, optional direction and restriction that a player takes on to temporarily add some spice to his otherwise humdrum slaughter of vorpal bunnies and blue slimes. Quests typically have very transient results, so they're also very shallow and zero-agency.
I think that it might be more interesting to approach this problem from the other direction. Instead of giving a player more freedom and then trying to compensate by reducing their agency, how about giving a player less freedom and more agency?
For example, a player normally makes a character. Picks a race, a hair color, stats, whatever. Instead, what if we presented the player with his choice of three characters. These characters are generated mostly randomly, but are created with ties to game events (questy-type-things). For example, you might get an elven archer, but the elven archer starts off well-known as the defender of a small village somewhere, and therefore has very different opportunities and NPC reactions near that area. Maybe another choice is a cybernetic commando who is known for the fact that he survived the first encounter with the Strekhakh aliens.
While the player has limited choices, the choices are actually more meaningful - have more agency - than the standard "race, stats, class" crap that players normally go through. It ties the player to the world proper.
Similarly, revamping quests to provide agency rather than distraction might be possible. Quests would need to be generated based on an algorithm rather than painstakingly scripted in, and they would need to have a long-term result based on the performance of the player in the quest.
The player is "linked in" to the quest network by the character he's chosen. A character who lived through the Strekhakh attack will (at least initially) have quests related to that (either the aliens or his performance during the invasion). The elf defender will probably have quests related to defending and coddling the town.
This quest-based mentality allows you to control how the game progresses by simply controlling the number, type, and importance of result from any given type of quest. If you want the Strekhakhs to be dangerous, than the best result of a quest is simply to keep them at bay.
Quest results could also compile into greater outcomes as well: if fifty people defeat Strekhakhs locally and grab random alien artifacts and stuff, the game itself can say "The Strekhakhs are being pushed back!" - not as a result from an individual quest, but from the overall quests.
The same thing is true of characters. If you want the game to revolve around Strekhakhs, offer more characters whose pasts involve the Strekhakhs. You can even introduce special plots by offering characters that have extremely strange backgrounds and therefore get very unusual quests and information.
The downside is that this kind of game would require a very robust method of creating and distributing quests. NPCs are the standard, but the NPCs would have to be able to refer players to other NPCs and so forth, which could get difficult. There are other options, they all have tradeoffs.
Anyhow, I think that limiting choice is probably a better choice than unlimiting choice. What do you think?