Word to the people who said that Ani slash tends to include the animal-morphing abilities. I am an avid slash reader (albeit not in this fandom), but animal sex makes me feel icky, and I just can't read it. Dude, I read one on ffn where Jake's tiger penis was creating erotic sensations for Cassie with its barbs. I don't know how that was in any way sexy; I just winced and had that image permanently engraved into my mind. And now yours, because really, that bears sharing.
As for pairings, I can see the whole sleepover make-out between Cassie and Rachel. I actually think some development could be done with hurt/comfort and Marco/Jake...with Marco falling apart and Jake needing to be stoic. I kind of see that around the beginning of the series--even before the series--and more of a role reversal during the end. I really like the idea of Jake feeling the need to provide extra emotional support for his best friend but remaining untouched and distant through it all, because he's not getting anything out of it.
I also like Marco/Ax. It can be really cute.
I've seen a lot of Jake/Tobias fanfics lying around and I've never really seen that pairing in my head. Mostly because RACHEL/TOBIAS FTWKTHX. It's hard for me to imagine Tobias thinking sexually about anyone else.
I saw a good post-54 piece where Cassie morphed Rachel and had sex with Tobias. Not slash, but unconventional and very dark.
That said, I really don't like Animorphs slash. This probably has a lot to do with the fact that relationships were not the main focus of this series. Rachel and Tobias are the best pairing ever and it's hard for most fans, it seems, to see them with anyone else. Jake and Cassie were also very canon and...stable? So we're left with these two relationships that developed in a war and really...mean a lot to a lot of readers. Playing around with slash or unconventional pairings is given very little room here, because 2/3 of our main characters are already involved, the majority of minor characters are evil or androids (whoa sex with androids). It's hard to break up--or justify cheating--when that's really not a plot point _at all_.
Even though Rachel has great chemistry with Marco, she'd never cheat on Tobias.
Tobias is madly in love with Rachel.
Cassie wouldn't go for Marco (ah!) or Ax, has no chance with Tobias and maybe could fool around with Rachel. Maybe.
In general, Animorphs is not very slashable. I read my BtVS/AtS slash by the truckload; those types of shows are perfect for that kind of thing. Animorphs is not set up so that there are valid alternate parings. (Unless you're in MM4, where Rachel dated Marco and that was WONDERFUL.)