For lifetime sales, I think it's going to stay quite close to how it is now. The more simple games like platformers, shooters and simulation games are the ones that are doing the best. The days of RPGs being the one of the biggest genres are long past; they're too advanced for the majority of the audience.
I think next generation of home consoles is going to be roughly as even as this generation in terms of hardware sales. None of the companies will manage to get a much bigger installbase than the others. They're most likely all going to hav to settle at sub 100 million or around 100 million levels. As a result, the next generation of Ps3/X360 games are likely to sell about as much as the Ps3/X360 games did. In other words: few games beating the 10 million barrier, with perhaps 5 or so games beating 15 million on the two combined. (This is assuming that next gen, Sony and Microsoft's machines are relatively similar, and Nintendo's is quite different. If S&Ms machines are very different, I doubt more than 1 or 2 games per machine could come close to 15 million.
As the Wii's successor will get far more competition from third parties, I'd expect close to none of that machine's games to end in the 25 million range (as at least 6 of the Wii's games will). Still, a fair bit of Nintendo's games should beat 10 million. Say, 5 or something.
But the 3DS, that's going to be an even bigger success than the DS. I expect that's where the main sellers of the next generation are going to come from. I wouldn't be surprised if it became the first machine to see 10, or even 15, titles sell 10 million .
Essentially, I'm predicting a bit of the same as this gen, just less huge games for the consoles, and more for the 3DS.
Among the big hitters (10 million ) on the 3DS, it's going to be 70-30 Nintendo games to non-Nintendo games, I'd guess. The games are likely to mostly be "non-games" (Nintendogs, Brain Training) and relatively simple games (Mario games, Pokemon), possibly with one or two more advanced games.
The next Wii should see it's top titles be Mario Kart, a Mario Platformer or two, possibly Zelda and Smash Bros and an equavalient to the "Wii series". This is Nintendo we're talking about, apart from the Wii series, they've hardly ever come up with a huge surprise on the software sales front.
The Ps3/X360 ones are the hard ones to predict. Every generation, the "most popular" genre from last generation has seen a bit of a hit. RPGs in the SNES era, and racers in the 6th era are the most obvious choices. Now, I'd expect something of the same to happen with shooters. We're either looking at an industry that's headed towards being "shooters and Nintendo games", or headed away from shooters. I think the latter is far more likely. There should still be some major shooter sales next gen, though, just not quite the same as this gen. There will also be more shooters (most likely), so each game should also sell less.
As for what will replace the shooters, I really have no idea. It depends on how Microsoft and Sony play it. I'd guess they might be headed down the casual road, so more Wii-style games from them too is a possibility.
If you're looking for the biggest openers, then that's essentially the same as above, but just ignoring the ones on the Wii's successor.
Fun fact: The Wii might end with 7 games in the 20 million range, but just 1, or possibly none, within the 10-20 million range.
