You're right, nearly anything or anybody could be made to fit the smash series, we've even had characters like Snake who was even more ''derailed'' from the series than all other 3rd party characters I've seen. And still he ended up being a great addition to the brawl roster in my opinion.
Even though this is very biased, I feel like the entire roster should at least have a form of battle experience. Which was at least somewhat true until they introduced these guys
(I also have trouble digesting
and
).
Yeah, Snake is one of the more different characters in the series, but I think he still fits well enough.
As for battle experience, there have always been characters with no real fighting experience, even since the original. In 64, there was
, who had only raced in a vehicle up to this point (the same applies to
, but to a lesser extent, since he had taken part in a different form of combat, that being space battles). Melee introduced
as you said, but also
,
/
, and
. Brawl introduced
, like you said. And now with Smash 4, we have
,
,
, and
. That's really one of my favorite things about the series: it adds characters you would never expect for this type of game, yet somehow manages to add them seamlessly and give them unique fighting styles.