From my experience, either allow multiclienting completely or prevent multiclienting entirely, no in between
You will end up hunting for cheaters and listen to players complaining about multiclienting all day long.
Its like the saying "Give them a finger, and they will grab the whole hand".
That would be my advice.
I'd also add to what Agony suggested, make something unique. The servers I enjoyed the most were the ones that were creative and offered a different kind of gameplay.
Keep it classic, but try to do things a bit different. Maybe new starter areas, change up the end game areas (why do people mostly pvp in Jungle/DD1? Add a few changes and make them PvP in another map, it could be refreshing).
I do not agree with the "no debuff" mentality though. Debuff lapis are part of the game and obtainable by everyone. If debuff lapis are taken out, sonic and flash lapis should be too, my 2 cents.
Also, do not even bother implementing Lv7 Lapis, EP4 is not meant to have those, for a good reason.
If you're interested in custom gear, do not add random stats and damage values. Closely calculate the percentage increase per tier to not end up with imbalance and overpowered gear.
If the difference from 50 gear to 60 gear is, say, 5%, then make the difference from 60 gear to your custom set also be 5% to make it seem more natural without overpowering.