What will you be chasing?

It'll depend on the individual rod the line you match it up with and your casting style, but in general

A 9 weight would give you a bit more oomph on the flats in wind for stuff you'd use an 8 weight for now.

A 10 will tend to give you more lifting power if you fish from a boat in deeper water, and will cast larger flies easier-- a great choice for throwing large menhaden and herring flies for striped bass here in the northeast, roosters in baja, mahi off shore, medium tarpon 60-100, and would be good as a backup on the flats with a crab fly for permit or a tarpon fly for medium to large tarpon that you might bump into while fishing for other stuff on the flats or if you also want to throw at some sharks. In freshwater it would be good for pike and chinook salmon In terms of versatility an 8 + 10 would probably expand your options more in terms of other species you could chase compared to an 8 + 9.

One thing to keep in mind is that there is generally a bigger step up in reels and backing capacity for 10 weights compared to a 8 and 9.

Reels for 8 and 9 weight rods are typically interchangeable and matched with reels holding 200 yds of 20lb dacron/micron backing

10 weight reels are generally designed to hold at least 200yds of 30 lb.

I agree with tailingloop too-- a 10 would probably be as heavy as you'd want to go, especially if you're blind casting all day---

11 and 12 weights have a much more limited application. If you do decide to chase big tarpon down the road you could use the guides 12 weight--- though you'd likely be better off using your 10 if you had one in terms of delivering the fly to get one to eat anyway.