Just so I establish that I do have a dog in this fight, my Deepkin collection is: 60x thralls, 30x reavers, 18x eels, turtle, 3x sharks, Aspect of the Sea, 3x Soulrenders, and all the other heroes x1.
I think it is probably totally doable to do what you are describing above. I STILL think the Deepkin should have gotten a couple more units by now. I very much dislike the release/whatever you want to call it strategy behind Deepkin, Overlords, Fyreslayers (though they got a bit of love).
I remember when Atia was first breaking rumors about AoS and one of the main ones was that GW would not be supporting armies beyond initial releases other than with small splash releases (and also that they would focus on monsters and heroes). If you think about it, the only army where this rumor has proved false is Stormcast. It is one of the rumors that bothered me the most, because it does not lend itself to long term concentration on one army. Maybe this is what GW wants. I guess probably they do want you to collect each new army as it comes out. But from the perspective of someone who collects as much to create a story focused group as to play, it sort of is getting old.
I guess we will see, as AoS is pretty new still. But I would really like to see true unit expansions for factions (and this includes old factions like Seraphon and Skaven!).