Dual Mode games are awesome but always challenge the game's integrity sadly.
I'm sure they could figure something out like keep offline data that checks itself again server data.
Honestly, it would be cool to have a few offline/online servers where you could overpower your character, mod freely, use bots, etc. Keep them separate from the normal servers.
Anyone breaking ToS wouldn't bebanned, but they'd be transferred to those servers where anything goes.
Dungeon Defenders did that (Mod friendly online/offline unranked mode where there were no rules or restrictions and a strict online only ranked mode), and I had characters in both modes. Sometimes it's fun to prance around as a gamebreaking god.