I'm not one for formaliy. I mean, my concept of "formal" is: my clothes aren't ripped(much), my shorts are clean and I'm wearing my black cross-trainers. Being an engineer, it's only necessary I do my job, not be a master of social graces. Not saying I'm some slob either.
From my reading, the Grand March is one component. I take it this must be a "tradition" thing. Traditions are great. Mine is usually "make Simon Cowell look like the useless wussy that he is", and when I say that, I make him look like the brightest angel in the sky when I'm working auditions, but that's a whole other issue.
I would say if that's your line of thought, I don't really see anything wrong with that. You should still do the Grand March, as that's a component or element all unto itself. Should you want some sort of formal old grand ballroom dancing, then I think that can be integrated into the event as well, which, depending on the attire of the girls, should be quite visually pleasant, especially when viewed from an upper level. No, I'm not getting all perv or something. I'm talking about the movement of the clothing as well as the pattern and movement across the dance floor. Shouldn't be quite 18-19th Century with the hoop skirts and corsets, but should still be visually pleasant. These would have to be held as sepparate activities within the event. But how many? You want enough to sort of keep things on the up and up, but also few enough to let the students have sufficient appropriate fun for the dance.
Waltzng is a dance, while Grand March is the entrance. Two things that shouldn't cross. They can occur in the same event, but not at the same time.
I think you wanting to change it up is not appropriate, by substituting a waltz for the Grand March, for example. I think maybe having an "old formal dance" theme might be better, and then have a few traditional dances AFTER the Grand Waltz.
After you find something usable, maybe give the students 5-10 different "Grant March" step sequences. Then let the students decide as couples or groups which ones from those choices that they will choose to do at the Grand March. I think the variety should prevent it from being boring. Yes, you'll have some duplicates in a row. If you're allowing for improvisation or students to make up their own, as long as they stick to the guidelines laid down and use good judgement, it should be acceptable. Besides, the girls will most likely be into this over the boys and the last thing the guys want is the girl angry with them!
Keep with tradition, and then maybe add to tradition. Those are my thoughts. Grand March first, add in some formal dances, but keep the rest more "modern traditional/norms". My guess would be that should be a success. Make sure you have the music you want played for certain elements comiled onto one CD for ease of usage for the DJ or sound company.