Whilst I'd want a large number of characters to be present, I doubt anyone would feel comfortable with Slade there, nor would I expect Snart to be present. They barely interacted.
An appearance by Slade offering his condolences may work, but he's not the kinda' guy to show up at the funeral I don't think. Those in attendance at the funeral should be actual friends of Ollie, or people that are close friends with a supporting cast member.
- Team Arrow (obviously)
- The Legends (specifically Sara Lance, Ray Palmer and John Constantine)
- there's really no need for any of the others, as they haven't had that connection to Ollie
- Team Flash (Barry, Cisco, Iris, maybe Joe?)
- Don't quite see Caitlin needing to be there, nor a Wells. Has Ralph ever even met Ollie?
- Supergirl - and Supergirl alone. Alex, John, Brainy (etc) haven't really interacted enough with Ollie to justify them being there.
Maybe a few guest characters too, like Nyssa? One thing that does make me wonder though (and I may be remembering this incorrectly), but isn't Ollie present when Barry disappears (in the future)? I seem to remember him being mentioned, or referenced, alongside Supergirl? If that is indeed the case, then either Barry is going to disappear earlier than expected, or the timeline is going to change. That, or we're all wrong, and Ollie isn't going to die just yet.