This story is a crossover for fallout but little the first one this one has no fixed crossover also the main character is from Fallout new Vegas and Fallout 3. But uses Fallout 4 tech explains and it is a little AU for all of these. It's been a very long time since I played fallout 1 and 2 so those events are all canon
The Lone Wander aka The Courier aka The Immortal Man(after all the cybernetic in him and the genetic engineering he had done on himself when he had investigated what the aliens had done to his adopted daughter) aka James Cameron. Has had enough forging peace between the Brotherhood and Steel and the Institute along with the Railroad had taking him away from the mothership zeta which had resulted in his adopted daughter Sally(who still had not age a day pass 12) almost being killed by one of the few remaining aliens and his friend Elliott did die. He hadn't really got on with the Brotherhood for some time now ever since Sarah had been killed in action against one of the few remaining Enclave holdouts. Not even getting into when he acted as a Courier out west when the was investigating a Mr House and the massive mess that it was he helped with.
along with finding a little robot called ED-E their. that he had heavily upgraded as when the local brotherhood chapter and the followers of the apocalypse both wanted the data it held he copied the data and gave it to both one after the other getting armor upgrades for the bot from the brotherhood and the followers of the apocalypse upgrade the weapon. then there was all the upgrades found in the divide that he implemented.
That wasn't the only tech he got from out west the energy to matter machines and their design from big mountain along with the their auto doc's, and hologram tech and all the implants and subdermal armor bought from the follower's along with the implants from Big mt itself. Along with recharger technology. Using the alien mothership he design the technology to work with the fusion cell's and core and combine it with the gatling laser(that can be modified into a heavy laser cannon aka a small version of liberty primes main weapon each shot cost 10 times that of a normal shot from the gatling laser ) as well as a laser rifle and pistol. He also design a plasma rifle and pistol that used fusion cells instead of plasma cartridges with recharger tech however he always had preferred laser weapons.
He had also design a power armor it looked like Enclave mk II power armor(fallout four mod) it used a mix of saturnite and durframe alloy and it held every upgrade that could be put on it however he was unable to put a Tesla coil or jetpack on it the Tesla coil as it messed with the recharger technology he inclued and as for the jetpack it's space was taking up with a downsized energy to matter(which has it own recharge technology and independent fusion core)converter fitted to the back for the armor to be able to build anything that has been programmed into it. Fitted to the side of the converter is his gatling laser (with gun sights, charging barrels and a beam focuser ) on the other side is a laser rifle ( improved sights, overcharged capacitor, recoil compensating stock, improved charging barrel and a fine-tuned beam focuser) and inside the armor on his hip is a laser pistol (improved sights, overcharged capacitor, sharpshooter grip, improved short barrel and a fine-tuned beam focuser), and a few plasma grenades and nuka grenades and his full shadow heavy combat armor with army fatigue under it with ballistic weave mk V.
On the medical side he had massively improve the stimpack by adding the mysterious serum that he found from that Lorenzo guy. it turned out to be his blood easy enough to synthesize it even without an energy to matter converter(mostly by adding a normal stimpack injection as the stem cell it contains copy the healthy cell nearby). Addictol(pretty common cure to addiction even if Addictol itself was rare now-a-day's) and Cure-all which he got from vault 87 from a miss nanny that he later help gain a synth body. It was pretty lucky no one got bit by any of the molerat's there apparently they had a disease that could easily kill someone young and cause permanent harm to an adult. and as the disease had no cure meaning it be untreatable without having to use the only know Cure-all in existed.
With everything packed up he look to his adopted daughter and smile they had found on the alien ship a dimensional transport device. And they were going to use it alongside their kind of pet robot ED-E(dog-meat had die a few years ago before he found out about cyborg-dog's and how to make them). Sally herself was wearing child sized army fatigues with ballistic weave mk V and had a laser pistol (improved sights, overcharged capacitor, sharpshooter grip, improved short barrel and a fine-tuned beam focuser) and a upgrade pip boy 3000 similar to his own allowing computer access to ED-E and the Energy to matter converter alongside everything that was able to be done on a normal super computer thanks to the increase in computing power by studying ED-E and the alien ship itself having the pip boy's store all the knowledge they had collect in this dimensional including schematic's of everything they had ever come across even if they didn't understand it. They had sent via teleport a message to the BOS in the capital wasteland as well as launched a beacon(it take about ten minutes to reach the citadel) to allow the brotherhood to take over the ship now that there was no one else going to be living there especially as they were going to destroy the dimensional transport device behind them.
As they prepare for the distortion of using the device he was happy he knew about the safe measures that make sure that they were transported going to be transported to a place that can support life of the type the device is transporting in the dimensional it transports the passengers to.
He smiles when he thinks of the adventure they are going on soon and hopefully to a place that that hasn't been hit with nuclear war.
A fresh start.
And done for this I would suggest the universe they end up be say stargate end of season 1, Battlestar Galactica anytime between the tylium asteroid and the battle of new Caprica, halo around 3-4 years before the fall of reach, Skyrim just after the dragon attack on helgen, rwby shortly before or after team rwby has formed . Babylon 5 could work but most of it's tech is around the same level or higher. But Mass effect is a good idea around the time that the Prothean beacon on mars is found.