Based on trends I see in funding, jobs, talent, buzz, etc., here's my list. It would be much smaller:

Portland: obvious, though many locals hate it.

San Angeles: it's kind of one city overall, but tech activity is concentrated in: San Diego (downtown area), Irvine/Costa Mesa, Santa Monica, Pasadena, and maybe downtown LA. I live near Irvine now and everyone I talk to says the tech scene has grown a lot and is still growing, and I see evidence of that myself. One of the strengths of this region (other than sheer size and being a few hours by cheap plane ride from SV) is something people like to bash it about: it's one big giant mass of sprawl with either many city centers or none depending on how you look at it. While that's not hot and trendy these days, it also means that this region can grow a lot without catching acute real estate hyperinflation disease. There's just so damn much of it and there are so many town/city governments that nobody can establish a NIMBY monopoly. It's not exactly cheap but it's not "$2M for a starter home" crazy and probably never will be unless you insist on the absolute hippest neighborhood or living within walking distance of a surfing mecca beach. Self-driving cars are going to absolutely revolutionize transit down here and fix the region's commuting problems in the next 5-10 years.

New York: already #2 or #3 depending on how you count it. Could grow a lot because it's just so huge and has tons of money available. Also has great transit to offset the real estate costs-- you don't have to live in Manhattan or the trendiest parts of Brooklyn and you don't have to have one car per person. Commuting from as far away as Connecticut and New Jersey south of the crummy areas is entirely possible.

Boston: Boston really should have been Silicon Valley in the first place-- the reasons why it's not are beyond the scope of this post. (I lived there for a while and could almost write a book.) If these things can be fixed they have more than enough talent and the city itself is very nice.

Some longer-shot wildcards:

Denver: it's cool and has appeal and seems to be drawing a crowd.

Pittsburgh: it's like Boston lite with numerous great schools but less expensive and less stuffy. It lacks many of the cultural problems that keep Boston from being Silicon Valley in spite of Boston's incredible talent pool. Uber and Google have added gravity.

Detroit: being kind of re-settled by an interesting mix of people. Things have happened there before and could happen there again. It's got a "cool factor" to it that other rust belt cities that have fallen on hard times just don't have. If I were 20 years old and in a different life-phase I would definitely consider going there, since escaping the real estate death spiral treadmill would offer a great opportunity to work on longer term "high risk / high payoff" projects. Think of $20k homes as a very light personal version of the DARPA fund for big ideas. (Assuming you have little fear of ghetto, which in my case is true.)

Toronto: close to a very good tech school (Waterloo) and UToronto is not bad either, a very big cool city, and doesn't have the real estate madness of Vancouver. Otherwise the latter might also make the list. (SF/SV can at least survive its real estate madness because it's so prosperous otherwise, but Vancouver doesn't have the muscle to overcome this handicap.)

I do think tech is geographically re-diversifying, so I wouldn't totally count out other places. But those are the ones I'd say have a good shot of developing into true "centers" as opposed to just places with some tech stuff going on.