I think we can all agree that it would be nice to have the content of the grimoire available in-game. (I've been especially enjoying the books of sorrow lately :D). So I was thinking it could be cool to have a new area in the tower which could be like a library themed place. The different existing categories could be on display in several rooms, which would result in more of a crossing of a library and a museum, maybe?
I also think some sort of personal area which you can customize would be very nice. Could be a house/room or ship interior where you can go with your fireteam.I think this could work well with the addition of trophies you get when completing certain content, which you could then use to further personalise. (I realise this would take a lot of extra development work, these are just ideas, of course).
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Perhaps the grimoire could be accessed at the library by The Speaker.