So I've been mulling ideas around off and one ever since I read the F/SN visual novel and watched Fate/Zero four years ago. I just discovered the new F/SN animated series and while thinking on my ideas I got hit by inspiration that made me want to actually start writing a fanfiction. My biggest question concerns F/HA and the Heaven's Feel route. I've looked up and down the Type-Moon wiki on info about Avenger, Bazett, and F/HA in general, but just reading a wiki is very difficult to piece together what's going on in that story. So my question is Does F/HA affect what happens after Heaven's Feel in any big, major way? I wanted to write a story of the 6th Holy Grail War where the Eizenburg family reactivates Heaven's Feel to open the path to the Greater Holy Grail again. However, I do not wish to just completely ignore F/HA, but since there doesn't seem to be a way to read it in English yet I decided to ask for help.
Another major question I feel I should ask is are there other series that are important to tie to F/SN in terms of writing a cohesive fanfiction? I do eventually want to branch away from just F/SN, but atm I would like to divert my free time to planning and plotting out this fanfiction and perhaps getting a chapter or two done while I still have this passion in me.
And third is just some help for thinking of someone to fill the Assassin role. For every other role I kinda thought of a list of potential candidates and then built the master's personality to work off of them, but I really want Assassin's master to be an Eisenburg mage who commands a legion of puppets and a few powerful homonculi in preparation for this war. Basically, a guy who's had enough of the Eisenburg participant in the previous wars failing (or disobeying in Ilya's case). I kinda wanted an Assassin who could possess the puppets or have some way to make the puppets stronger, but I can't really find any legends that seem to fit. And yes, I know this is more of Castor's territory, but I have already decided Castor's, Saber's, and Rider's roles in the story and will not budge on them.
I thank everyone who takes the time to respond. I have tried my best to keep to either stick to the rules from F/SN and Fate/Zero or think of a really good reason why a rule got broken.