Was Cicero the Adoring Fan?

He wasn't.

The passage in Cicero's journal is probably a reference to the adoring fan from Oblivion but it does not mean Cicero was that adoring fan. There have been many arena champions over the years aside from the Hero of Kvatch and many would have attracted fans.

Cicero's Journal also includes the dates of his assassinations. His journal states he completed the arena contract in 4E 187- only 14 years before the events of Skyrim. It is theoretically possible the Hero of Kvatch survived and was somehow still the arena champion (if s/he was an elf or a vampire for example) but this is a moot point- Cicero was not The Adoring Fan.

It's also worth noting that the adoring fan in Oblivion looks completely different, sounds completely different, acts completely different and is a completely different race from Cicero.

Some speculate so, yes. And via thorough research it is easy to understand why. Cicero, being a master of disguise, could have potentially fooled the Hero into believing he was a Bosmer child. However, there are some deductions:

Cicero is mad, so I believe if you were to turn down the Adoring Fan, he wouldn't be so patient.

Also, you can potentially just brutally murder the Fan, which means he wouldn't even be alive in the next game.

Last and most importantly, Bosmer, Altmer, Orsimer, Dunmer, Falmer, etc. live much longer than Men. They are not immortal, but live for quite a while. So the Adoring Fan could be alive by the events of Skyrim, but Cicero couldn't be. If I am correct, our beloved Cicero, correct me if I'm wrong, is an Imperial. Which again, correct me if I'm not right, don't live as long. In order for Cicero to be alive during the Oblivion Crisis, he would have to be over two hundred years old by the events of Skyrim. Imperials, being Men, don't live that long.

So, it's a good theory, but it's just implausible. But I could be wrong.

Unless, of course, Cicero is just another alias for something greater...