At first I wasn't interested in amiibos at all, but I'm more an more interested by the idea of getting a few of them. For a dedicated Smash fan like myself, I like the idea of getting figures of characters who would otherwise have very low potential to get merchandise based on them. For the next wave I'm particularly interested in getting Pac-Man and Wario because their respective amiibo look very good, maybe Robin as well - his face doesn't really look like his one from the official artwork, but at least it doesn't look ridiculous, and otherwise the amiibo looks good, especially given how Robin is one of the most detailed-looking characters. Well, unless the final product looks worse... Not interested in Charizard Lucina and Ness though. Unfortunately I can't preorder them, so I'll try to get them as soon as possible before they become too rare and/or overpriced (I don't like the idea of paying more than €20 for getting one, and I definitely don't want to spend more than 25€).
So my prediction for wave 5 amiibos are:
We already know that wave 4 will release in April in Japan, and I could see the Mewtwo DLC being released for everyone in May, as well as wave 5, making the timing perfect. Mr. G&W and R.O.B. are definitely going to be rare, and Olimar as well, but maybe not as much as the other two. Maybe. On the other hand Bowser Jr., Jigglypuff and most notably Mewtwo will sell like hotcakes because well, Mario and Pokémon merchandise always sell well. Always. Unless you're some obscure and/or unpopular character such as Stantler, Garbodor or Il Piantissimo.
And for wave 5 exclusions (and thus definitely wave 6):
Unless wave 5 releases after E3 then I'm going to be genuinely surprised if he's in that wave. He's going to be released after the Zelda U announcement imo.
Same as above, but for the new Starfox. Because it's 99% likely that neither Zelda and Starfox are going to get more details revealed about them before E3.
Saved for the reveal of a currently unannounced Metroid title which is going to be revealed at E3. Though more wishful thinking than anything else.
Saved for after the announcement or the new gen VI Pokémon games based on Kalos and featuring Mega Zygarde and Mega Kalos Starters. Again, wishful thinking.
I feel that they'll keep the most iconic retro for the last wave. Though maybe the average people will not recognize/want him because he's not doing his infamous laugh pose lol.
Not real justification here, but he will add some needed iconicness factor to the wave 8 roster. Because he's still Mario anyway, so he's going to sell well.
Possibly even more of a wishful thinking than ZSS and Greninja, but a new Kid Icarus title is possibly early in development currently and going to be revealed at E3. Though it's rather unlikely and I could totally put them in wave 5 instead.
And finally, one based on the first DLC newcomer announced at E3.
Oh, and btw, I'm expecting this wave to be announced at E3, one month later wave 5's release, so their reveal will coincide with the E3 announcements.