When you say 'custom', it really means a lot of things:
custom quests, items, modifications, monsters, etc etc.
As for items, I say there should be NONE. Personally, for lowrates, it'd be best if there were no custom items. Be it for aesthetics use, or disguises, whatsoever. NONE. As for quests, yes. Infact, custom quests such like boosting EXP for an hour to the whole server, or an event for that would be good. Since lowrates are all about PvM, MvPing and s***. Or, even quests that doesn't require you to do the job-change quest, but that's just some examples anyway.
As for modifications, that's up to you(server owners) to come up with, lol. Monsters, there's pros and cons towards this. Monsters for PvM(aka leveling or w/e, novice monsters), are fine imo. But no to MvPs.