Oh dear (diety name here), please do not make things depend on other things unless they literally are the only thing that could realistically be expected to make it work.

Even if Open Media Player is supposed to replace the main package, it should be left to the end user to make that decision unless a horribly un-work-around-able reason makes it a PITA to make that happen. Even if your Open Media Player is the best media player that ever walked the earth, someone will want to have both on their system, and if they want it, they should be able to have it. So no ____ conflicts with _____ just because some dev didn't think normal people wouldn't want both, no ____ depends on ______ unless the latter actually HAS TO have the former, and if you're going to do that you should also be watchful of other things that other developers might make that fulfill that dependency. (EG, Kernel Power Settings shouldn't depend on JUST Kernel Power, it should optionally depend on kernel power, UBoot-for-kernel-power, and any other package that flashes the power kernel to your kernel partition. Etc.)

So I say the latter is fine, however, the ideal solution in my opinion is just exterminate all the 'dependencies' that aren't actually necessary when you make the CSSU (IE, no, my FreOffice, FM Radio, or whatever else I want, does not depend on my having the preinstalled hildon-application-manager on the device, and don't you dare try to tell me otherwise Nokia), but don't make it so that the new stuff replaces dependencies - someone might not want to have a media player on their device, at all. That's odd, and that's unlikely as hell, but it's feasible enough that if someone does feel that way, they should be able to do it without having to muck around with .deb control files for every little thing.

So, my recommendation is, kill the unnecessary dependencies, don't make any new ones unless they literally HAVE TO be there, and everyone developing the FOSS replacements to the closed source apps, I say do the same thing: Don't stick any conflicts between the two unless they have to be there.