So I'll start by saying I really like the Armguard. I don't even play mid very often and I like it. The armor/AP combo was lacking in versatility with just Zhonya's and with the season 3 bruiser mid shenanigans, the SA is awesome.
That being said... a clever and cool item that only builds into one item? I think it can easily branch into some other things.
First, and I'm not going to get overly specific with costs or anything, is Fiendish Codex. Adding the initial growing AP and armor with AP/CDR could help out a lot of spell reliant champions. I haven't thought of a further stacking possibility, but you could easily toss in a blasting wand for more AP in the combined item, or a Kindlegem for more tankiness/CDR (I think the latter option would work quite well on Gragas, for example).
An even better option, IMO, is to combine SA with the Glacial Shroud. I know we recently got a new GS build, but that was really overdue anyway. That was another great item that had only one combine option until the Iceborn Gauntlet. I think as a primarily Armor item, with CDR, mana, and AP thrown in, a combination of these items would work well with champs who like to build tanky but have spells scaling off AP and not AD.
Lastly, I think it would make a good hybrid with the Twin Shadows (it even sounds great... SEEKER'S armguards? That's gold, people). The only problem with this is it feels like you'd need to combine it with a watered down version of the Twin Shadows. But the result would be a well rounded AP/armor/mag. resist item - it could be very good against hybrid damage champs who are otherwise tough to itemize against. I would suggest making a weaker active though, like reducing the "seeking ghosts" down to a single one.
Discussion: Kinda obvious. What sort of items do you really hope to see the new Seeker's Armguard build into?