The only thing against Geno is his third party status. Otherwise, he was a shoe-in. Don't believe me? Here's why:
- Rumored SMRPG Remake (very likely)
- Rumored SMRPG Sequel (less likely)
Both of those would be footing for Geno to get in as it would mean he could be placed in for advertisement the same way Roy was. The remake is highly likely. The sequel, not as much. And both of the rumors have a semi-official root so it wasn't just some guy making up those rumors. It's highly likely we'll get a remake. I wouldn't doubt the sequel rumor either.
- SMRPG was the first Mario RPG
Face it, Mario RPGs are starting to become a big part of the Mario franchise. There are more Mario RPGs than there are 3D Mario platformers. Paper Mario was insprired from SMRPG and was originally intended to be a sequel. And the Mario RPGs have been widely successful and well-recieved. Being in the game that kicked that genre off definitely increases his chances.
You can check this out for more information on its impact on the gaming and Mario industries:
http://www.smrpglegacy.com/index.php?p=v7-smrpg10impact
- 5 mentions on Sakurai's Journal
Implies Sakurai was interested in the character. The music alone would have been a very nice addition as it's widely remixed. If Sakurai shows interest in a character, then it's likely for that character to get in, in some form.
- Easy/Unique moveset
His moveset would have been very unique and would be easy to create. Also, it would have brought a different play style to Brawl much like Olimar did. The simple fact that he would be unique and impossible to be a clone should have increased his chances beyond that of other clones like Dr. Mario.
The ONLY thing that destroyed it all was his third party status. Otherwise you can gurantee that he would have been in.