But given that they didn't go that way with Martha, the only place where it's been properly fully canonical is the DWM comic strip. So here is me burbling about it, partly because it's International Day of Femslash, partly because I really, really want Izzy/Destrii fic to exist and am too lazy to write it myself, and partly because someone on the anon meme asked where they can get hold of it. (Answer to which is: I don't know, if you're talking about what I think you're talking about, but there are Amazon links herein. It's worth it!)
I was a latecomer to the DWM comic collections for really very lame reasons, but a little under a year ago I chewed my way through all four Eighth Doctor ones in less than a month. They're bloody brilliant, and a large part of why they're brilliant is the companions. Unlike Sam in the books and Charley in the audios, Eight's main companion in the comic, Izzy Sinclair (or "Izzy Someone", as she calls herself before she resolves her issues about being adopted), is neither blonde (well, not completely; she has highlights) nor in love with the Doctor. She has got the inevitable late '90s SF character thing of being an SF fan so they can make self-referential jokes, but I've never really minded that.
And the reason she's not in love with the Doctor: she's gay, and was always planned that way. (It takes a long time for it to become text, but it was always planned, which I just love.)
Izzy's a resident of Stockbridge, a town you've never heard of if you haven't read the comics but if you have it's where the Doctor goes all the frakking time. Anyway, Izzy is friends with a local UFO spotter named Max who Five got caught up with this one time, and after they get involved in some business with the Celestial Toymaker (who I find yawnsome, I'm afraid, but at least he gives them an excuse for the Scrabble tiles spelling out the name on the cover and stuff, which is wicked cool) and ends up coming on board the TARDIS afterwards. This is the first story in the collection Endgame.
A couple of adventures later (still in Endgame, collections-wise), in an adventure called, erm, "Tooth and Claw", they run into Fey Truscott-Sade, a British secret agent from the 1930s, who the Doctor has met in a previous unseen adventure and who is asking for help now. There are hints of mutual attraction between Izzy and Fey almost from the very beginning; there's one lovely panel where you can actually see Izzy go a little bit weak at the knees (I will do my perennial "waah, if only I had a scanner I would be able to show you this" thing here and then shut up, but suffice to say that any time I am describing things in detail it is because of the picture/words exchange rate being so high).
Things get very complicated very quickly, however. The Doctor is in a bad state at the end of T&C, and has to be taken home to Gallifrey. Fey manages to do this by reading the manual (leading to a cooling between her and Izzy as Izzy feels somewhat unneeded in comparison), but things on Gallifrey are themselves in a parlous state and it's only with the help of matrix-construct Shayde that the Doctor gets through it all. He appears to regenerate (into Nick Briggs!), but this is actually a fakeout: Shayde has taken the Doctor's place so that the Doctor can get one over on the Threshold, the people responsible for Ace's death on the moon in yet another of her divergent fates.
The Threshold, thinking the Doctor is weak after his regeneration, strike -- they have been using an unwitting Fey to monitor things ever since Tooth and Claw; turns out she was only able to read the TARDIS manual, written in Gallifreyan, because of their interference. The fakeout is revealed, and the Threshold turn out to be basically Shayde's cousins once-removed. In the final battle, Shayde is badly weakened; cut off from the willpower of the "Matrix lords" on Gallifrey that powers him, he is helpless. Until Fey steps into the breach and merges with him, offering her willpower instead. The merged Fey/Shayde depart, and the Doctor and Izzy go on to more adventures.
(The next collection, The Glorious Dead, is not so relevant to the femslash-i-ness so I shall not go into depth here. It is, however, awesome in its own right so you should totally read it. The epic story which makes up the bulk of it and after which the collection is named has got it all: shedloads of Doctor/Master, Cyberleader Kroton the only Cyberman with emotions, and a plot that prefigures vast amounts of stuff from the new series, particularly the end of S3. There's also the return of Grace in the lead-in story.)
The third collection, Oblivion, is the first in colour and the key one for all of this stuff. Izzy meets Destrii, an alien fish-person princess-on-the-run, who swaps bodies with her and attempts to get on board the TARDIS. The Doctor realises, however, and goes back for the real Izzy in Destrii's body (unfortunately, Izzy's original body with Destrii in it is seemingly destroyed). Thus begins a fairly long arc in which Izzy is trapped in this new body, gradually trying to come to terms with it. But Destrii's people are looking for her and kidnap Izzy by mistake, taking her "home" to the desolate planet of Oblivion.
The Doctor, desperately trying to find Izzy, calls on Shayde and Fey (who have gone back to her home time where they are now fighting in WWII, with occasional "I want to use our powers to kill Hitler"/"Noes, timelines" arguments) for help. They first manage to find the not-dead-after-all Destrii-in-Izzy's-body, and then take her home. Lots of complicated stuff happens there, but the key thing is that eventually they manage to swap bodies back and in the process getting a glorious double-page spread of seeing into each other's minds and back stories (there's a lovely parallel of them both watching Star Trek; Destrii's uncle tapped Earth tranmissions, you see), in which we find out more about Izzy being too shy to ever say anything to the people she was attracted to, and so on. (All of this makes me ship Izzy/Destrii like whoa, incidentally, though there's no explicit canon backup for it. But there's so much potential, between the bodyswapping, the potential for angsty hate stuff [I've glossed over the bit where they have to fight in an alien arena here, but *ahem*], and the seeing-into-each-other's-minds thing.)
The whole process leaves Izzy with a new confidence. As Fey leaves, they have a chat about it that ends with Fey kissing her and it's just one of those lovely moments that makes you melt.
(Izzy then leaves the Doctor, getting the Ben-and-Polly "returned on the same day you left" ending and having resolved her issues about being adopted, though she does appear very briefly in Eight's final story, "The Flood", after which the last Eight collection is named. The collection starts with several "breather" comics after all the heavygoing stuff that's just happened, but when the Doctor meets Destrii again and she becomes the next companion it really kicks into gear. The title story has future Cybermen and Destrii completely kicking Cyber-arse, and is the one I put at the end of my personal Eight continuity over Gallifrey Chronicles and whatever the audios eventually decide to do, because it has such a lovely ending.)