I don't like the idea of prestige for building, but I liked the idea of prestige as a marker of contribution for a SG or a currency to earn. I don't really know what would be a good use of a SG only currency though. Having something to show for time played, outside of Inf, which can be relatively quickly capped out, and xp, which after Vet lvl 100, doesn't do anything.
My reasoning being after you have so many completed characters, they need something to do after they've got all their Enh and Incarnates. Badges and Inf are more of a singular toon project. Something to influence what toon you choose to play that day. As it stands, aside from wanting to play a certain combo, there's no reason to play a non-farmer after they're geared out so a lot of characters gather dust.
Maybe a custom vendor like P2W, could sell things like badges, costume unlocks, things that'd make a toon feel "prestigious".