Chrom can echo Roy who aren't even in the same (outside of FEH) so I could totally see that happening.
Eh, not the same situation. Roy just had the same bodyshape in Smash that Chrom had available model-wise.
The theory is that Roy being effectively a cross between Ike and Marth is why Chrom uses Roy as a specific base. Which fits why Chrom didn't make it in 4. All we know is that he was a fan favorite and made it into the game because of that. Chrom had to use a ton of new animations to make him work due to Roy using a different hand animation for he he swings his sword(backhanded instead of fronthanded... though I probably have that said wrong).
They don't need to even be from the same franchise for cloning/echoing. They just need to be able to potentially use the same abilities. Ganondorf can legitimately punch and kick with magic(for instance), though he was going to have the Tech Demo Sword, but he also had the right model at the time for a clone.
Echoes on the other hand have to have the same proportions(or barely a difference), and in some cases, it worked out another way. Lucina was not the same proportions as Marth originally, but got added to Smash as a costume first, being modified enough, so she could be split off later. They do sometimes modify proportions slightly, but it's so rare it's best to look at it as "do they have the same proportions right now?". In this case, Bass does have workable designs to go with, even if the one in Smash is not at the moment. At least to tie into the other thing.
Bass could theoretically be an echo of Megaman.
That being said, if they ever did go for a Megaman Echo, Protoman would probably be a more sensible, easier character to clonify.
Not necessarily. Bass' worst factor is that not all the models are the same size as MegaMan. But we know he has models the same size. Protoman's Shield is a major thing to deal with, and they have to actually either make it mostly an unused asset(just appears on the model) or heavily change up his moveset to remotely use it. Bass doesn't have this issue cause he has the same core bodyshape as MegaMan, with at most a slightly different headpiece(which doesn't affect proportions at all in this case). All 3 easily use the same moveset, and were designed to be pretty much identical outside of having some unique special moves only in their particular games. Also, Bass was designed to be a straight counterpart to MegaMan, even having his own dog that does nearly the same stuff(armor options too). Protoman is similar to MegaMan, but wasn't meant to be his counterpart, but a brother with similar abilities. Roll is another example, but she's... I think too different bodyshape-wise? She clearly could be a regular clone, regardless of customs. But she doesn't normally use the variable weapons systems like the others do.
Hell, X and Zero use the variable weapon systems, but differently. X copies, while Zero adapts it into a new weapon.