I'm taking a couple of sick days off from the Holocron. Mostly resting, but also working on the fanfic. This is a big fight at Croft Manor, which was originally thrown in to put a little action into a chapter of archaeological discussion. But said chapter ran long, kicked the fight scene into its own chapter, and so that had to grow larger as well.
In the original Legend/Underworld canon (aka, the first trilogy of games produced by the studio Crystal Dynamics) Amanda sends the Doppleganger into Croft Manor to steal back the Wraith Stone. My head canon is Amanda tried earlier but was stymied by the house security (one of the in-game excuses for using Doppie is that she passes for Lara on a retinal scanner).
And in my fic, this first attack takes place while members of SG-1 are visiting.
See, this is what is exciting about cross-overs. Like AU stories, you've established a change. It may be a small change (there are a large number of fanfics in which "Harry" Potter is born a girl. And for whatever reason, many of them get rather lemony as well -- usually involving Hermione. But nevermind!)
The fun is that changes cascade. It is almost never a matter of replacing each occurrence of one name or gender or hair color or whatever but otherwise retelling the same story. Instead, events diverge, proceeding inexorably from that first change.
Of course one reason is that fanfics are by their nature fix fics; they are an author's response to another's work, and they may chose to comment on or to change elements of that work. This ranges from exposing some of what one might consider poorer choices in the original, to completely changing the spirit. (This may be why in, say, Harry Potter fanfics, Voldemort rarely survives much past the Triwizard Tournament.)
This is a dialog that happens within original works as well; many works of science fiction and fantasy were written to ridicule or overturn existing genre conventions. Or to introduce elements that had been previously lacking (c.f. feminist science fiction, the New Wave, etc., etc.)
The other reason is because this is the fun. The fun of all the alternate history and What If stories. Make one change, whether it is Lee victorious at Gettysburg or the Enterprise showing up in orbit around Caprica, and see what follows.
(Personally, for me this is practically the point. The original canon Harry Potter universe is all about how special magic is and how useless muggles are. Making Harry...err...Amaryllis Potter the (other) young ward of Bruce Wayne means that muggle skills and muggle ways proceed to kick ass.)
Unfortunately I'm not really doing this. There's no specific linkage of events that causes my Amanda to send a full team of mercenaries to get the Wraith Stone back. If this does come out of anything, it is from something that far predates Lara's awareness of the Stargate program.
It comes from the fact that in this universe, both canons are true. At least, as much as is possible to reconcile them. The Stargate and the Goa'uld have always been part of Lara Croft's universe, waiting for her to explore the right tomb to find them. And Amanda and the Wraith Stone, Excalibur and Mjolnir and Natla have always been part of the Stargate, SG1 universe; again, just waiting for the heroes of that show to discover them.
Oh, and one other thing. In my particular universe, the fusion I've chosen for this specific work of fiction, people aren't idiots. Actually, in the SG1 universe military people are surprisingly competent (of course "by TV standards" is a low bar, but still). And so are scientists, by and large. What's been the biggest problem for me in the majority of the fic is my efforts to admit history, archaeology, paleontology, geology, evolutionary biology, etc., etc. to the competence club; to assume that archaeologists would probably have noticed if, say, the Great Pyramid of Cheops at Giza had gone flying around the galaxy ca 3,000 BC.
In any case, this chapter brings the general military competence of the SG1 universe, as well as my general slant towards improved realism (but only when and if compatible with telling a good story) to an armed attack on Croft Manor.
It ain't going to be Home Alone. Stupid tricks don't actually work against even poorly trained gunmen (well, not often enough to risk). And you don't fire off automatic weapons indoors without someone getting hurt. I mean to maim one of my cast before this is done.
Ah, but sigh; I have no real battle plan, no good overall flow. Just a bunch of character "bits" I want to do. Half of which are, well basically, those same Home Alone kinds of gags. The cake is getting divided at least four ways here.
So none of this is unfolding organically from the original assumptions. Instead my choices are thoroughly constrained. I'm leveraging the architecture of the manor itself for every bit of plot convenience I can get from it, and as well forcing each essentially isolated encounter to fall the way I want it to fall instead of developing in any logical way.
One day. One day I will write a proper AU/cross-over, in which I really do defend how each battle came out different because of my original nail.