By 2020, I predict, there will be a "standard" gaming platform. There will be multiple consoles, yes, and many of them will have their own proprietary format, but they'll all support this basic platform. It might upgrade every once in a while (but hopefully not). Maybe it'll be DirectX or OpenGL-based, or maybe it'll be something out of Nintendo, or a GameTap or Steam console. We currently have DVD as a video format - likewise, we'll have a single, basic, unified gaming format. All console manufacturers or game makers will pay a modest fee to license this format, and include it on their machine.
It'll play a games in that format. It'll load them off whatever the optical media du jour is, or it'll download them from the Internet (there'll be a browser and a specific game code). People will use it as a platform for many of their online games, as well. If the platform isn't GameTap or Steam itself, then at least both GameTap and Steam, or something like it, will exist here.
This is basically the alleged "Phantom" console, right? Wrong, for two reasons - one, it's much more open. There will not be just one manufacturer of it, and the manufacturer won't be the publisher either. It's just a platform. Two, it'll have to happen at a time when the console market is crashing. Console wars are essentially format wars, and three contemporary formats is two too many.
You can go on making present-style consoles afterwards, consoles that run proprietary formats and eclusive games, but hopefully that'll die out after a bit due to sluggish sales. See, this unified format is what's necessary to break into mainstream.
In Japan, Nintendo is turning the DS into a unified format for mobile gaming, though I don't think handheld gaming is a big enough idea to attract the mainstream, and needing a specific brand of hardware is likely to frighten off the timid mainstream consumer.
For mainstream, non-Flash-portal-style gaming to succeed, there needs to be the potential for hardware knockoffs. Which means that the original developer of the hardware needs to primarily take the role as licensor of the technology, let the hardware manufacturers compete on overall quality rather than "features" (from a business perspective, every game released on a console is an optional feature, and exclusive titles are selling points), and let developers not have to worry about choosing a development platform (since it'll automatically be cross-platform).
PCs already manage this to some extent. Now imagine the ease of consoles (guaranteed compatibility, no console lockdown, standardized input) with the openness of PCs (broad price spectrum, niche titles, ubiquity), plus whatever new fancy features invent in the next fourteen years (especially with regards to online play).
One format at a time. The death of exclusivity. That's what gaming needs to go mainstream.