There is current construction work happening at Pier A right now: a new ground-loading facility is being built (future gates A1-A10) which will be used mainly for WestJet Encore flights and possibly Central Mountain Air and Hawkair flights as well. Currently there are 3 temporary ground loading spots at A (A9, A10, A12; A8 is closed for apron expansion for the aforementioned project) and Encore already has to use B22 for extra Encore flights. Central Mountain Air and Hakwair predominantly use B23A through B23C (and occasionally B26), but all three airlines should fit in the new A, freeing up extra space in B22 to B26. Also, the temporary A tunnel blocks B11 and B12, so when the tunnel is gone, these gates will likely come back.
After that, I project that the B pier will be renovated/expanded. Once the props at B pier move over to the new A pier, B19 to B26 would not be used much anymore, so they could tear it down and expand it sort of like how they expanded C. Right now B27 and B28 are *rarely* used, and with B11 and B12 likely to come online again, these can hold the flights that were originally at B19 through B21.
For an international expansion, new gates could be built as an extension of D. Gates D59 thorough D63 were never built but they fit in that area without the need to move any of the taxiways. And if this is done, D73 and maybe D71 could be permanently re-designated E gates for more US-bound capacity. Alternatively, the gates can be slightly reconfigured and a E74 could be added for more narrowbody space.
In addition, in the recently new A-B Connector, gates B14 through B17 seem to have the ability to receive international arrivals (there's the overhead walkway which will eventually lead to the customs area). It's possible that some of WS's international flights (e.g. to Mexico or the future London flight) could operate from B instead, further freeing up space in D for international airlines. IIRC, B14/15 could "merge" together to form a widebody gate as well.
[Edited 2015-11-06 19:35:51]