You can forget about low rate if were talking about l2j. So its gotta be mid (they last longer than high rates).
If you can guarantee a ddos/bot/phx etc protection then you should focus on the features. Keep customs to a minimum and start polls about xp/sp rates! Retail buff slots and a reuse delay for mana potions (sadly today's l2 community can't play without them), so there will be more balanced pvps! Also imo its better if it is hard work to get S gear, it will keep people busy longer! That means low drop,rb drop, and more importantly quest rates! Also do not put customs npc with enchants or other unbalanced crap, and do not implement an aio system or anything like that! This is a good starting point before you can go on with the details...hope I helped you a bit!
Edit: Yes, Interlude would be the best choice!