Oak
If it is Oak, that is sick. Since Gary is Oak's grandson, Ash is the same age as Gary. Go figure.... Oak must be a LOT older than Delia, I think they are just friends. UNLESS, he returned to the forest, and encontered Celebi again. Time Travel could, in theory, change everything.
Normal Trainer
Quite likely, and it seems from WPM's source that this is true. Guess its not 100% certain though. That could have been gospel, or just a possibility.
Ho-oh
80% certain it is Ho-oh. I am sure Ho-oh can either transform itself or transform others. It could easily be true, and if so maybe it was "guiding" Ash when he saw it originally. Why else would Ho-oh be on Route 1 at that exact time? Ho-oh is truly sacred, and Ash is obviously supposed to be "special". This is most obvious in Pokemon 2000.
A Celebrity (Real World)
It seems the kind of thing that possibly could happen. You never know. I totally disagree with this theory though.
Mr. Mime
I wouldn't think about Mr. Mime too deeply. "Mimey" is just an overactive friendly cleaner. "Mime, Mime. Mis-ter Mime".
Giovanni
Possibly the most talked about theory. I don't agree. He is the bad guy, IMO and that is as far as it goes. This isn't Star Wars.
Brock
Someone suggested this at SPP. NO, just NO.
May
I'd say this is not very likely.
The Trainer from the Battle (Nidorino VS. Gengar)
Unless this has been revelead already who it is, it remains a possibility.
A Game Boy Advance
Marketing, maybe.
Pikachu
MAYBE. Just maybe, Pikachu is a reincarnation of Ash's dad. The original one possibily having died.
Fearow
AKA Mew. A test of ability and determination, and sacrifice (Pidgeot). If you don't know which Fearow I mean, ask, I will tell you.
Lance
Some people have suggested it. I can't see it myself, but there are connections.
Other
I could have missed them completely