That's how you can make the game last.  So far the most killer app for these amiibo are smash bros. and everything else pales in comparison.  At least it makes a worth wild game to play amiibos with.  So what does this mean?  Another $4.99 goes to nintendo for both 3ds and wii u, but it's not like you get a new character every week, month or even the next 3 months.  It will be polished, detailed, and finished when it's ready.  Plus, even when the NX comes if it's for home console or mobile, they may still have backwards capability if not just a digital download of the game.  I don't really need a new smash bros. in the next 10 years if it's still coming out with new characters. 

So, good idea or bad idea?

theres too many of them!

Of which?  Smash Characters or Amiibo characters or both?  Most amiibo released not for smash bros. are already smash characters.  New comers would be Toad, and the Inklings from Splatoon which could just be a skin character so not two characters.  And we already know that the only reason why people buy new smash bros game because of the newest roster of characters.  This game play for the most recent smash is fine, I'm not really buying a new smash for the new game mechanics. 

I don't think you realize just how long and hard it is to make a single unique fighter...

And I think you're over estimating on how long and hard it really is?  A year, half a year if they start now on a new one right now?  Keep the DLC far and few inbetween.  We don't have a Bayonetta Amiibo and amiibo aren't capatible with her game anyways so not every game new game will have an amiibo.  But really what other amiibos have been announced that don't already have characters in smash brothers?  Epic Yard have yoshis.  Mario party 10 has Mario, peach, Luigi, bowser and toad, which is really just toad as a new character.  Splatoon can be 1 character that changes form from human to squid and boy girl skins. 

Well........that's a ton of Animal Crossing characters....

With that logic there would be Boo amiibos, Gumba amiibos, Roll amiibos, Tails amiibo, Ashly Amiibos, Crom Amiibos, Cutman and gutsman amiibos, etc...

If that were the case, R.I.P wallet.

What's $4.99 every 6 months.  Or if you only buy for one version $3.99.

Bad idea because there will be too many characters