Church should definitely have Youth Groups, but that doesn't mean they get a separate service or get to leave in the middle. Even the little ones need to stay for the entire Liturgy.
As for Liturgy, if you look at the past 2,000 years, while the Liturgy has changed, it hasn't been all that much, at least for that length of time. You aren't going to get a modernization.
In fact, I think our Liturgies need to be done fully, not cut short in any way or breezed through. In addition, we shouldn't just leave quickly after its over, if possible, a Church should have coffee hour, then Sunday School/Youth Group time.
As for "Youth Group" ideas, you have the OCF for college age students, and those in college towns should always have a chapter if there are enough students. For the Antiochians you have Teen Soyo for the teenagers. Then you also have programs in some Greek Churches for the youngest ones such as an Acolytes group for boys, or a Handmaidens/Myrrh-bearers/Altar Guild group for little girls.
Families should be encouraged to spend more time at their churches as able. If its a big enough church, a lunch after the service should be provided every Sunday as a potluck. For the adults, it could be good to have a catechism class, a Sunday School class for kids. On top of that, if its big enough, there should be individual groups such as Teen Soyo, OCF, Acolytes, Myrrh-bearers and other such groups. For adults you could have a men's night for men and philoptochos/women's night for women.
As for worship, you don't really touch the worship service and its structure.
Kids aren't leaving our church because it's too "traditional" they are leaving because their parents aren't instilling in them need to go every Sunday and dedicate your whole life to Christ, even when out of Church. There are so many parents out there who could bring their kids every Sunday, but cave in to themselves or to their kids if they don't feel like going, and what does that say to the kids?
Also, the other big problem is ethnno-centrism. The idea that you're Orthodox because you're _______ ethnicity doesn't lead kids to truly understand and appreciate Orthodoxy, and does, in fact, lead to lower attendance rates as studies have shown. Ethnic identity is important, but you aren't Orthodox because you're ______ ethnicity. You're Orthodox first and that ethnicity second, unfortunately so many families don't get this and see attending Liturgy as an ethnic ritual like dancing, celebrating national holidays and such.