With the PS3 having a really small amount of exclusive games, the Wii being 'too casual' and the 360's numerous mechanical flaws, I think a well-made system with a strong library could be quite competitive. Since Sega is a fairly major game developer, boasting a fairly long list of quality titles (although sadly, many have been in hibernation for quite some time) I think it's a possibility that they could take advantage of this. However, it would've made much more sense to do it sooner - even one or two years ago.
Putting their games on the Wii store would be a double-edged sword in this situation. On one hand, they have a competitor with rights to sell their old games - on the other, they now have people remembering those old games. Which is pretty nice marketing for them in the case that they do produce a new console (especially if they plan to do sequels to some of their older games on release), and they could use these figures as a gauge to measure what remains of their foothold in the market.
They have a solid list of titles that they already own and can build off of, that will also leave competitors hurting with the loss of them.
But this isn't going to happen. Because Sega's marketing is absolutely awful, and has been the downfall of the company for over fifteen years.