I think this should depend on how the characters develop and based on what the characters are ranked. If there is a maximum level, then there's not so much a need for a reset, since there is only so far that the top players can go to become the best, after that, it is just a battle for who gets the best items, spells, or whatever you have in your game.
If there is no level limit, you may want to put a time limit instead, which is basically a reset every so often, like you said in your topic.
Personally, I wouldn't like to lose everything each round, but, there are some ways to make players look forward to new rounds:
Provide an incentive for the players. Maybe players who have been in previous rounds get bonus stats or EXP, etc.
Let users pick a different race/technology route each round, and once they pick it they need to stick to it. Make sure each race/technology route has unique features, so every round they can try something new.