Well, there are still 3 more levels of ranks yet to be released. So I wouldn't count anything out just yet. GC Seal cap will probably get a raise to 60-75k and we'll likely get one of those levels in 3.0 (read: 3 ranks in the next tier up, the other 2 tiers probably will be available in later expansions). New GC gear, likely for optional leveling purposes and new materials for the GC crafting as the OP mentioned. The final set of GC armor for this level.. don't expect anything fancy. It's likely the uniform your respective GC OIC Quartermaster and Administrator wearing, and ultimately will be probably fairly useless aside for people that want to use it as glamour.
I expect 3.0 area FATEs to probably give a bit more seals than current high level FATEs, and/or all FATEs gaining a boost in seals. Then again if useful mats are sold in the GC, things might just stay the same/similar to as they are now.
That's what I think though.
As for what I do currently, yeah.. pretty much load up on about 50 ventures at a time (when I run out/low) aka. 10k seals. Make them back up with high level dungeon gear drops that I have no use for. Sometimes spend seals on gear if it benefits something I'm leveling up.
Added: Another thought crossed my mind.
What I would really like to see, is for tier I-IV materias to be made available to be purchased with GC seals at that new max rank as well. There needs to be something done to make materia more accessible and this would prove immensely helpful in reducing the ridiculous prices that grade IV materia have become, as well as easing the relic upgrades from being such a gil and/or timesink when it comes to acquiring the materia needed for the Novus stage. It would give GC seals another quite relevant use as a type of currency.
I thought Crit/Det IV prices were high on my server back when I was doing my Novus, yet several months later after I've long finished prices of those materia have only risen by another half of what they were when I was doing the Novus. I was totally taken aback when I actually checked it out one day. I thought if anything, after the initial Novus rush, prices would decrease and stabilize, but that never happened.
I didn't even go full crit/det or buy all of my materia and I still sunk roughly 4 mil into my relic on that stage. If I had wanted to go full crit/det with today's prices I would likely be spending something like 10-15 mil if I were to try and buy them all.
I've only recently slightly gotten into leveling up some crafting classes (started in 2.3), but I can already tell I'm not going to be in it like the hardcore crafters. Even here, it's too expensive to get materia for melds. I have been playing since beta, and the most gil I've ever had at one time is 8.2 mil. Back when I started Novus I had about 5 mil. Finished Novus with around 900k left, wasn't cool, but I'm glad that's all that it took me. It's a nightmare to even think about doing another relic. So I think having materia available to be purchased with GC seals at this stage (3.0) would be ideal.
I kind of also expect them to add another tier of materia V or even a second tier of VI. At the least add materia V, but have that only be available through Spirit Bonding since it'd be new.