It occured recently to me that, since the train to and from both wastelands is only two weeks, the Lone Wanderer could call in quite a few favors they'd gained from their time in D.C. to help with the upcoming battle. Or, alternatively, just pay a few of them to help. The changes would be mostly cosmetic, as with all of the 'additional forces' in the battle. (You can't fail as long as you don't die) I'm rather sure a few of these ideas are extremely possible to pull off. Others...less, but I'm optimistic. Just ideas so far, so feel free to post critiques/suggestions.
I need more ideas for Caesar's Legion support. They're hard to sell to some of the factions if the NCR is on the other side!
Here are the ideas I'll be looking into so far:
(Listed by perceived difficulty, easiest first)
1.) Mothership Zeta Death Beam [All Four Factions]
REQ: You completed Mothership Zeta, obviously.
Because why the hell not? You've got a death beam. Use it. I don't actually know how this would work outside of an automatically firing ARCHIMEDES II. Maybe show it blasting away things off in the distance, around where the Legate's Camp and Fortification Hill is?
Possible Bonus, thanks to Sir Swerving: Your surviving crew joins the fight, because they have no freaking clue about the modern world besides what you tell them.
2.) Talon Company [All Four Factions]
You'd suggest it, and depending on a few other variables (Faction reputation, Karma, Barter, logical state of faction economy, possibly involving "Beyond the Beef"? etc) you'd either have to shell out the substantial amount of caps, or the facion would do it. Nobody really cares one way or the other about Talon Company. You pay them, they show up and fight.
3.) Paradise Falls [Legion, Yes Man.] (Maybe a stretch...)
REQ: You didn't kill all of them.
Paid the same way as Talon Company. Slavers. Talk to Eulogy. They show up, and spew Mesmetron (Hah!) fire along with lots of Chinese Assault Rifles (if you dumped a bunch of them at their store)
4.) Powder Gangers [All Four Factions]
REQ: NCR/you didn't wipe them out, Neutral+ Reputation
These guys are idiots. They'll fight against the NCR, because they're not thinking ahead. They'll fight for the NCR, possibly on the behalf of House/Yes Man. If it's for the NCR, they're pardoned (since they'll just be cannon fodder, like the Khans). For everyone else, they get killed off. Once you get to the Legate's camp, if any survived, you can kill them to ensure that they don't terrorize the Mojave in the ending slides.
5.) Fiends [All Four Factions]
REQ: Motor-Runner Alive, Fiends not hostile due to "Aba Daba Honeymoon".
Same situation as Powder Gangers, ending slides included.
5.) Pitt Raiders [NCR, Yes Man, House]
REQ: Sided with Ashur
Note: If you have any kind of Brotherhood of Steel faction joining the fight, Ashur himself will not show up to the battle. Otherwise, he'll fight alongside you.
You're a citizen of the Pitt, and one of Ashur's most trusted. You've got a job for his army, and they're itching for a fight. He gets public exposure for his steel mill thus more profits and growth, and gets to help kill people from a real slave empire. Ashur is a man who wants to rebuild civilization, and if the Legion is trying to burn it down all over again, that's not ok. It may require lying to the NCR about exactly what the Pitt is like.
6.) Riley's Rangers [NCR, House/Yes Man with Speech Check]
NOTE: Cannot hire both Riley's Rangers and Talon Company/Paradise Falls. Considering looking into hiring Talon for Caesar to increase Riley's Rangers damage output because they hate them so much. That'd be neat.
You'd suggest it, head on back to D.C. and, provided that you completed the quest and everyone survived, they're on board! Riley thinks you're one of her own, so what's a two week trip to Vegas to fight off some misogynist slaver scum for a country she's never heard of? They owe you big, and would probably jump at the opportunity to help with this.
Alternate Suggestion, by Sir Swerving: Convincing them to relocate to the Mojave, as there is a larger market for good and honest mercs. They can help more people this way, especially since the BoS has the Super Mutant problem under control/eradicated by 2281.
7.) D.C. Brotherhood [NCR + Mojave Brotherhood Alliance Only]
NOTE, thanks to Shardoom: If you've allied with the Remnants, you have to pass a 100 speech check to assure Elder Lyons that they really aren't Enclave anymore and they just want to do some good before they're gone.
They owe you everything after Broken Steel, assuming you didn't nuke them. Suggest it, get Elder Lyon's approval (provided you gave him the information about Vault 87 so it's safe to assume there are less Super Mutants to deal with) and you've got roughly the same size team size as the Mojave addition. D.C. wears T-45d, though. Airdrop via Brotherhood Vertibird, just as the battle starts, I'm thinking.
8.) Lyon's Pride [NCR+ Mojave Brotherhood Alliance + D.C Brotherhood Only]
REQ: Sarah Lyons Alive, All Lyons Pride Alive, Listened to Kodiak's story, completed Broken Steel, won the pot for Gallows' real name, took the Power Armor at the start of "Take it Back!", completed "Galaxy News Radio", Very High Karma, Idolized by Mojave Brotherhood...probably a lot more, so it's not automatic. Maybe Veronica stays with the Brotherhood?
Sarah and the Pride probably wouldn't stand misogynist slaver scum. If it's the assault on the Purifier all over again, they're ready for it. Replaces equal number of D.C. Brotherhood troopers you'd normally get, and are much more effective in combat.
9.) Liberty Prime [NCR, Only if already enlisted both #7 and #8]
REQ: Turned in 100 cameras and 100 sensor modules, or 200 of one, to Scribe Rothschild
-The goddamn crown jewel of all of your efforts in D.C. It's been four (or nine?) years. Liberty Prime is up and running, better than ever. You want to take the dam, you take an army. If you want to hold the dam, you get the giant fucking robot. For lore purposes, you'd have to airlift him in so Arizona isn't completely on fire by the time he makes it to the wrong side of the Colorado river. Would require the entire Dam walkway being in the same worldspace, so he can walk across.
EDIT: The Hoover Dam is actually, surprisingly, just in the WastelandNV worldspace. Each Cell is also labeled very, very aptly. :D The barricades are there to slow down the assault for the player's benefit. I think. Anyway, ALL of the sandbags would have to be removed for him to actually walk across. So just use him for cover, I guess.
Also, no more football nukes. That would cause WAY too much friendly fire for this fight and probably blow up the dam. I know this is possible, because they did this in Fallout 3 over a longer and more complicated stretch of terrain with lots of script dependencies tied to his physical advancement toward the Memorial. He HAS to take down the barriers for the quest to progress, so this is totally possible. Probably. Hopefully.
God I hope it is.