PC, PS4, and the Xbone are the 3 major platforms right now. I may not care for PC gaming, but it's still a big deal. As for the 3DS, I've never been altogether impressed by its software, which is a pale shadow compared to its predecessor and doesn't suit my tastes like the Vita's games do. As for why I made 2 recommendations, that was partially because everyone's tastes are different and partially because in my mind the Wii U is a good secondary console to complement a PS4 primary console. Nintendo bothers to release a worthwhile game on average about once every 3-6 months, and inbetween those releases you have the PS4's constant deluge of titles.
PC gaming isn't even a thought for me either. I can't think of anyone I know who is a serious PC gamer. Usually, it seems that everyone I know just plays one game on PC like StarCraft, WoW or Civilization and had a console for the rest of their gaming. I suppose you're right since I always hear of so many people buying games from Steam but I can't think of the last time I heard of a PC game making big sales since WoW and its expansion packs. At the same time, a PC is a bit different animal too. Most people will buy or own a PC for a lot of other uses and being able to game on it is a bonus or after-though. Generally with a console, the primary reason of buying it is to play games. Casuals are more likely to buy a PC for other needs as a game console is more of a luxury item. Thus, my question still works in that regard of what would you'd recommend as a purchase for such an item.
It's interesting that you mention the Wii U as a secondary console or compliment to a primary console like PS4. I really think that is Nintendo's only play these days. Heck, the idea may have even started back in the 64/PS1 days. With the Wii, the idea definitely took off with Microsoft even promoting that thought. With the Playstation and Xbox systems often being quite similar in what they offer with a few trade offs here and there, Nintendo does stand out as more different from them. Although they are probably out there, I can't think of knowing anyone who owns say a PS4 and XboxOne only and not a Nintendo system. Generally, if they own both, they'll also buy a Wii U and just own all systems in that case. A Sony and MS only gamer is I think more of a rarity. I'd think that an MS or Sony gamer would be more likely to buy a Nintendo console for a secondary purchase myself.
With the Wii, I think it was able to reach the sales it did because of that idea of a secondary purchase and because it was different with its unique controller and the idea of motion controls. At the same time, it was able to create many brand new consumers who only bought it as a primary console and who later got bored of it. If Nintendo hadn't gotten so many non-gamers interested in the console, I still think it would have sold quite well but it would have probably been more of a close race with MS or Sony and may have even ended up second in the end depending on how long Nintendo extends the life of the system in that case of lesser sales.
If Nintendo had released a console very similar to 360 and PS3 without motion controls but just their regular output and quality first party titles, third party support would probably dry up again. It would be very hard to make it a secondary purchase as the price for it would most likely have been much higher like the PS3 and 360 were at launch. But by having a unique experience, it encouraged other gamers to buy it for a different experience.
While a secondary purchase is most likely not Nintendo's goal, it almost has to be. Even if Nintendo were to release a console exactly like the PS4, why would a long time Sony gamer make the switch over? Because he can play Nintendo games? If a person wanted that, then they'd buy a Nintendo console. In that case, Nintendo would have more market share, Sony would have much less and third parties would have to develop more and support the Nintendo console. With very comparable specs, there shouldn't be any complaints about making software for the system and third parties can continue to make the software the way they always do without having to make compromises to develop for a Nintendo console. Thus, everything should be sunshine and rainbows and work out perfectly.
Yet, if a Sony customer really wanted Nintendo games, he'd probably already be buying a Nintendo system. Most likely, they are buying a Playstation because they expect certain titles to be on it. Final Fantasy, Metal Gear Solid, Grand Theft Auto, Call of Duty, FarCry, Batman: Arkham something, etc. Very few of those titles have appeared on a Nintendo console in over a decade or at all. This customer is now supposed to take the leap of faith that they will all or say 85% of the titles will come over and appear on this new Nintendo console? What happens if he takes the leap of faith but others don't and so the new Sony console still ends up selling more and having a larger market share? With a history of third parties constantly cutting support of a Nintendo console through the 64, Gamecube and Wii U eras, why take the chance of buying the Nintendo console when that could happen whereas its practically a guarantee all those titles will appear on a Sony console? The intital slow sales of PS3 didn't deter third parties and they kept making all those games for it then.
That's the issue Nintendo faces. How does releasing a very similar console to the competition convince gamers to jump ship to them along with third parties releasing their big titles on the system and staying with them for the long haul?