First you have to figure out why people left.
mirforme n chasey werent here in the early days. Choose any or more than one of these
The drops were a mess
The server got hacked and all the files released everyone told to change password. This worried many
Wizards werent the same so they all went
Testers said they reported many bugs that werent sorted before launch.
Some hit the soft cap very soon and they didnt want to level past 44 with level 30 items n skills.
There were lots of cheats, gold buying farming and that at the start which went unpunished for too long.
The Tiny XP acpi tweakers went away .
An Apex person told me they all left(100 ish ppl) because the GMs screwed up their hunts, wouldnt fix SW discount, took SW gold from guild store and capped the income from SW without notification.
People didnt like things being changed on the fly.
No one liked curse immunity potions being in the gameshop.
Highly visible things like the Luck system needed attention FAST but didnt get it.
Suggestions to encourage return
Convince everyone that the new host is secure and safe.
List how many items and skills are ingame since new drop files came into game.( if this possible)
Show that the new exp requirements are a steady increase
You could have a returner package. For accounts that have been inactive for 3 months. Exp pot bundle, quest kit with 2 rings 2 brace and and unbreakable stone which disappears after a month