Evil organisation (this could even be the TSAB if you want) kidnaps Vivio for her inherent ungodly powers, Nanoha tries to get them back legitimately, Vivio somehow DIES.
Final 7 episodes are filled with Nanoha no longer needing any more friends and straight up getting revenge for her lose. It starts with her hunting down the organisation with extreme prejudice. Everyone is with her from the start, but as it progresses to this point it becomes clear she's going too far in order to enact her revenge. Trying to stop her only pushes her further away before it ends on Nanoha berserker packing everything in her way and a final episode tear jerking battle between her and Fate who tries to bring her back to sanity.
Adapt it as you will, I just want Nanoha to freak the fuck out. I was hoping for it in StrikerS when Vivio was kidnapped from her, she was clearly pissed off for a change (the events rarely directly effect Nanoha, it's usually someone else's problem she's helping with) but she held back.
Bitch needs to lose all her restrictions and Raging Heart, who is sympathetically linked to Nanoha's emotion with her... but SevenArcs wouldn't do it.