Based on feats, Killer is below guys like X-Drake, Urogue and maybe Apoo, but if you're looking at portrayal, him and Kid, are meant to somewhat mirror Luffy and Zoro, so he will probably rise together with Zoro towards the end game.
As for others that could have been included:
- Marco and Jinbe are veterans from the previous era.
- Drake is secretly a marine.
- Apoo and Hawkins are on Kaido's side.
- Yamato isn't a pirate yet.
- Sanji and the other strawhats, no doubt have great significance going forward, but they dont really stand out like the Worst generation do, and I doubt they want to, considering that they aren't really concerned with climbing up the ladder.
Also, I think when Lady Toki mentioned 9 shadows she meant the Scabbards, I doubt other pirates will join in the fight. Either way, whether you think Killer is weaker than X character, by the end of the fight, he will be one of the 5 pirates leading the generation, excluding Blackbeard.