The following is a plot bunny that whacked me over the head during work and refuses to leave me alone until I give it some attention.
Sometime after the events of Sailor Moon, the Senshi wind up in an engagement with a new enemy who uses a type of magic they've never seen before and kicks the asses of the entire team with it. Saturn in particular gets beaten up pretty badly, but just as the bad guy is about to finish her off, reinforcements show up in an unexpected form; Takamachis Nanoha and Vivio swoop in out of the sky to arrest the bad guy, who turns out to be an Interdimensional criminal the TSAB's been after for a while.
The Nanoha timeline intersects with this fic in the years between StrikerS and the ViVid manga. Nanoha is on an enforced vacation to heal the damage she did to her linker core during the battle with the Saint's Cradle, and she and Vivio are on Earth visiting the rest of the Takamachi family. During this vacation, Nanoha gets a call from Main Branch HQ tipping her off to the presence of the criminal in her area (Uminari is just south of Tokyo for the purposes of this fic) purely for advisory purposes, but Nanoha decides it would be a good opportunity to give Vivio some experience with live combat under controlled conditions (i.e. Nanoha can step in and bind the guy if he starts to escape or seems likely to hurt Vivio really badly) since she's set on becoming a bureau mage like her two mamas.
So Nanoha and Vivio arrest the crook, but Saturn is so badly hurt that the Takamachi twins teleport her and the Senshi back to HQ with them for medical attention. During Saturn's recovery, however, the nature of her massive world-destroying power comes to light; the bureau is naturally quite concerned about a girl who can blow up planets by swinging a glaive around. The TSAB cannot, in good conscience, allow this girl to run loose with no training.
Those who've read the Nanoha novelization may have picked up on hints that the bureau forcibly recruits untrained mages for the sake of teaching them to control their powers, and this is what the Senshi immediately think the bureau is trying to do until Nanoha tells them otherwise; Hotaru just needs a magic tutor to show her how to force her magic to work under more a heavily-defined set of controls than what her Senshi form provides, in other words a magic system that is more complex than what she is currently using. And it just so happens that Nanoha is both a master of Mid-Childan magic and happens to have a magic teacher's license from her time as a combat instructor in the Air Force. She's also got an awful lot of free time on her hands since she's not teaching classes or flying around kicking ass for the next few years
That's right folks, Sailor Saturn is going to be taking magic classes from the White Devil herself and most likely getting to be friends with Vivio at some point. Attention all bad guys, you may now start gibbering in abject terror.