For sure this could be great!
I also feel it's quite hard to pinpoint the Necrons now however in terms of card design. With only a bunch of White Dwarfs and two codex to work with FFG should be able to make a start however the most recent Necron codex is such a step away from the original fluff I personally have no idea how to handle this subject currently. Not to say I have anything to do with design but I do feel it's sort of important to know the background in order to handle this.
I do think that the 5th edition codex would be more used for Conquest LCG so I fear we'll not see much of the great C'tan but much more have a pharao like feel to them. Tomb Kings , if you will.
Cards that have been popular in other CCG that have a good recursion tech could however be used for this faction. I also feel that we can freely say that the avarage Necron and Tyranid card will be more powerfull as the average normal card. This is because of the limited pool and no options to other factions.
So a 1 drop that is 2/2 with 1 command and 1 shield for Necrons should not be out of the question and at the same time I expect a 2/1 1 drop with a single command and a 2/1 token for the Tyranids (both can't have wargear).