So far the Nexus for each seems Player focused rather than Gamemaster, but the Game Compendium for Pathfinder allowed me to easy filter all Large Level 4 Aberrations (only three in the material I have access to), for example.
I would be surprised if that happens. There's no reason for Hasbro to do that at all. I'm excited because third-party publishers (like the ones I work for) might be able to have something as slick as DnD Beyond (by the original team behind DDB), but open to everyone.
Here's what I value most on DDB, and therefore what I hope to see on Demiplane. My ranking is:
Crucial: it's a dealbreaker if this feature is not included.
Important: a strong consideration, but not a dealbreaker.
Nice: definitely makes the experience better, but I don't use it that much.
Meh: probably important for some folks, but doesn't do much for me.
These are all opinions!
1. Crucial: Full integration of all texts, meaning that rules are all hyperlinked together across books, character sheets, etc. From what I can see, Demiplane has this. The integrated, interactive character sheet is particularly vital, so since they have this they are already in good shape.
2. Crucial: Content sharing, so that my players can access whatever I need them to. DDB is ridiculously generous about this (I currently am sharing almost all of WotC's publications with around 30 people).
3. Important: Encounter Builder/Operator, so that I can quickly build, save, and run encounters, tracking initiative order, stat sheets, etc. This saves me hours of work and greatly enhances gameplay, so much so that it is almost crucial.
4. Important: Completeness: all official publications are available.
5. Important: Modifiability, so that I can add my own creatures, items, etc. This may extend to being able to add sub-classes, new abilities, etc., but it's the former two that I really use.
6. Nice: Interactivity, such as automated dice rolls, doing the sums for players, etc. I work with a lot of new players who don't even own dice, so this is nice to have, but lending out dice isn't hard (and I strongly prefer using them) so this doesn't factor much into my rating.
7. Meh: bling. DDB is always giving me fancy character sheet templates, backgrounds, fancy virtual dice, etc. I honestly could care less, as long as the basic character sheet and texts are organized and presentable.
From what I can see and you are telling me, Demiplane already offers the most important thing, the integrated character sheet, and I assume that this means there is some form of content sharing? If the only thing currently missing is the encounter builder/operator, then they are already in great shape. The fact that the person behind DDB also made Demiplane is extremely promising.