BlazBlue:
Now Lelouch would want to take over the N.O.L but the problem is Izanami, Terumi/Hazama and Relius who have high mind hax resistance. Plus Relius' own mind hax is greater than Lelouch's mind hax so he could turn the tables and mind screw Lelouch and turn him into his puppet. Now Relius has Lelouch and is curious on Geass and began ripping into his mind and try to find the Collective Unconscious to gain a Geass or maybe a Code to gain some form of immortality. Now Relius has the code and maybe he gives Hazama AND Terumi both a Geass? Oh crap I think Lelouch turned things worse...
That's one side of a bad ending for Lelouch and for the BlazBlue world but on the other chances, if he meets Ragna I think they might get along since both of them have a strong bond and would do anything for their little sister (Nunually for Lelouch and Saya for Ragna). Since Ragna is out destroying N.O.L bases on his own Lelouch would probably recruit him into his group after some arguing since their goals align such as Ragna wanting to destroy the N.O.L and Lelouch wanting to make a better world. Thus began the new version of the Black Knights, for more of the members I can see that Lelouch would realise how outmatched he is and who knows finds Kokonoe they both strike a friendship (of sorts) and since Lelouch shows his Knightmare Frame where Kokonoe gets interested and puts in Ars Magus into it as an extra back up device. This would be a second battery for the Sakuradite as such Lelouch has more firepower to combine, plus Kokonoe is a more sassy version of C.C and Lelouch would get along with her (
Wiggle eyebrows). This would mean Sector Seven is joined in with the Black Knights that includes Makoto, Mai, Noel, Lambda and Iron Tager. So far as the series progress I can see Jubei and Ragna training Lelouch in fighting though in Ragna's case beating the crap out of him until he gets stronger. Plus lets say when the Take-Mikazuchi is destroyed by Ragna like in canon in Chronophantasma I can see Lelouch getting the remains and placing them into his Knightmare with the help of Kokonoe creating a VERY powerful weapon.
Then somewhere down in this what if Lelouch meets Naoto Kurogane and Es he would be intrigued by the Boundary and would like to know more of their realities as such he learns a bit more from both of them, though Es would give cryptic remarks and Naoto would be clueless but Lelouch would try to Geass someone like Es but it fails though Naoto might be controlled to tell information though Lelouch can't Geass him again. Of course if Lelouch is around Naoto then Relius drops a lore bomb where Naoto's sister Saya is the originator of the Fifth Prime Field who is the mother of Ragna, Jin and Saya making Naoto their uncle causing Lelouch to be interested in the whole Murakumo stuff and the Prime Fields where he tries to find more about them with help of Naoto being kinda like his Suzaku if he never became his enemy. Then when Lelouch learns about that Es in her timeline knew Nine the Phantom and Celica he would be curious and would probably manipulate Celica's naivety to figure more about Nine and her past. Of course I can see Lelouch teaming up with Carl and figure out the Boundary and learn a few key things about it.
Though after Izanami and Terumi/Susano'o is defeated I can see Ragna getting the True BlazBlue and letting Lelouch rule over the world by shoving the world of Code Geass into the BlazBlue world so that way Lelouch can see everyone again. This began a tear jerker since Ragna just gained his family back Jin, and Saya (Since Noel, Izanami The Origin and Mu-12 are parts of Saya) and Lelouch would never forget Ragna.