Yeah, I have been collecting Wii for a few years, and it definitely has a very underrated library that will skyrocket in price in a few years. Its already happening on some titles though, games like Pikmin 2, Kirbys Adventure Wii and more has seen a steady increase lately, and Im sure more will follow.
About handheld collecting, Jeremy Parish just started a youtube series on the Game Boy - so look forward to increased prices also there! (Guess I need to grab those Final Fantasy Legends-games and Castlevania 3 now before its too late lol).
But PS2 is amazing right now for sure. The price of the big hitters on the system, Devil May Cry, MGS, GTA, Final Fantasy etc, is ridiculously low compared to their quality, and most of the still good but obscure jrpgs are pretty easy to get for now. Guess its time to refocus a bit.