I already have a pair of Focal Aria 906 bookshelf speakers and a Denon X3700.  I’m looking to start building a surround setup in my gameroom.  I don’t really have a place for the 906s other than this room, so if they’re not utilized here, then they’d either end up in storage or on the market. These are the options I’ve been tossing around:

1) buy stands for the 906s and use them as L/R, buy the matching Aria Center channel and add a sub.  Add surrounds and height speakers later on when budget allows (most cost effective option)

2) Buying a new pair of towers (sub $3000), adding a center and sub.  Adding rears at a later date.   

3) Buying open box Aria 926 towers at  below $3000, buying the matching center, and trying to incorporate the 906s as rears.  Also would have to buy a sub (least cost effective option)

I would say that buying a sub is almost mandatory in your scenario unless you get a really hefty set of tower speakers.  The Focal towers would not really do well here because they are not effective at deep bass. 

If you bought a different set of towers and center, you could likely still use the 906 for surrounds.  I have found that the front 3 speakers need to be exactly the same sonic signature (i.e. same make/model).  However, the surrounds don't necessarily have to match 100% as long as they are close.

Even though you "could" use the 906 for front left/right with a sub, I would still go for something different.  Either get Focal towers or a different brand bookshelf that does bass/midbass really well.  The Focal just don't have bass/midbass strength.  The Focal tower speakers do "okay".

Even though you will have a sub that is likely crossed over at around 80hz, there is still a LOT of bass/midbass coming out of your left/right speakers between the 70hz and 150hz area.
So what you're saying is that it probably makes more sense to look elsewhere eventually than try to make a pair of $2800 Aria towers fit

For a while I was leaning towards SVS Prime Pinnacle towers and a demo Ultra center and SB4000 available locally
If you are looking at $2800 Aria towers such as the 926, you will probably be just fine. I’m saying that the 906 bookshelves will not have a strong enough midbass for left/right (my own opinion).

The SVS will definitely have stronger bass than Focal (even with Focal towers), but they are not going to be as refined or as high resolution. SVS uses polypropylene drivers for woofer/midrange and they are not as clean/clear sounding as other drivers because the poly material has a lot of flex.  It's up to you for what is more important.

For subwoofer, I would look at Rythmik subs first. I generally like ported subwoofer better because they have a thicker/meatier sound.
If you want to consider a really refined high-resolution tower with good bass "for the money", maybe look at Elac Carina, or even better Elac Vela.
Using the 906es as rear speakers seems a bit overkill and a little wasteful to me.  I think you were smart to consider just going with SVS.  You could get the Tower Surround package with their PB1000 Pro sub for about $2100 less whatever you could sell the Focals for and have a very good (read the reviews) and matched complete 5.1 system for very little incremental $.  I particularly like their center speaker that’s a true 3-way design with a dedicated midrange driver located beneath the tweeter that provides more even L/R dispersion than the more common 2-way MTM designs (like the Aria center BTW).  Anyway, that’s what I’d do FWIW.  Best of luck. 
@soix  do you have an SVS setup or just like their products?  A local store is selling an SB4000 for $1000 andan Ultra center for around $400. 
I just have their SB2000 sub, but if you read reviews of the speakers they — like the subs — get consistently glowing reviews, and as they sell direct they offer a lot of performance per $.  They also have excellent customer service and offer legit risk-free trials, so they believe in and stand by their products.  What’s not to like?  For HT purposes I’d consider snapping up the Ultra center and sub as they seem like pretty good deals and add Prime towers and surrounds unless Ultra speakers are in the budget.
I ended up finding a pair of nice  trade-in Aria 926s from a local authorized dealer for less than the price of a pair of new SVS pinnacles but it was a tough choice 

Going to pair with an SVS SB3000 and Aria center and try and use the existing 906s as rears

 thanks for all the input