Few ideas along a new server with ladder seasons:
1) New ladders on Diablo 2 also came with new items sometimes like ladder only runewords, new weapons/armors/events etc. If you added say, a few new weapons and armors per ladder reset then people would play more to get them to take to non-ladder.
2) The reset button would likely have to be reworked to prevent people from just resetting their bot from non-ladder to ladder and have their accumulated trophy points go with them (if we keep those).
3) We should call energy clan score just for nostalgia lol. It could work the same way if you wanted, I just like CS more xD
4) Try out free for all modes like total war etc. and other test ideas before pushing them to the main server like blue was would be a good idea.
5) Could change up the systems entirely towards PvE/Co-op if you wanted instead to see if more people would be interested in a PvE/Co-op only server with little to no PvP. Other games have this, they normally just call it a casual server. Jut no PvP whatsoever. People may say it's boring but a lot of MMORPGs are focused like this.
6) You could also make a PvP only server that gives people a certain amount of time to make bots then they all fight automatically at the end of the month in a tournament or can fight each other normally. Something like that.