I think it's still too early to consider that. The repository that is used for phpBB 4.x mods/plugins will have to interface with the forum solution on phpbb.com, which means it'll need to work with 4.x in order to work. So that means we really can't do much until we at least get a semi-stable framework of 4.0 (I'd say a few months before a Beta is starting to look possible). Then we will have to decide if our current solution will work (Titania is being designed so we can hopefully get it to work quite easily with 4.0 with relatively minor modifications) or if we need to build something new.
4.0 is years off before we get a Beta, so I think we should focus on other more immediate things for now.
At the least wait until you see Titania replace the current Mod/Style database