Let's take a situation that will be the majority of the WoW community. Uldar is release and there is no new hero or valor gear introduced. People get the new gear on their mains. Why would anybody, other than fun or rep, do heroics at this point? You can't really argue that people need it to gear up because as soon as you hit 80 on your alt a 10 or 25 man gear Uldar group can bring you along to a 25 man Naxx run and epic you out in a few short weeks.
What happened at the end of TBC was just ridiculous. The badge system got out of hand when people could turn in 150 badges for a SWP equivalent item. So I don't see people being able to turn in 200 heroism emblems in favor of a conquest item.
The only thing I can come up with to keep heroics somewhat alive is offer a conversion of some sort. Nothing game breaking but something that can fill a small void. Offer special mounts you can purchase.
Examples:
10 Heroism = 1 Valor
10 Valor = 1 Conquest
Mounts = 200-300 heroism
The conversion I used as an example would be a fair ratio, IMO. People won't be able to abuse this since a 30 conquest item would require 1,500 Heroism. And at a 4 emblem per instance ratio that would take 375 heroics to get one item. This would be more of a filler. When 3.1 goes live the number of emblems will be:
4- Obs
4- VoA (New boss in vault)
16- Naxx (15 bosses + 1 extra from KT)
15- Uldar (Assume 14 bosses + 1 extra from Yoggy) **Please note extra boss on hard mode might drop 1-2**
Per week you can net 39 Valor and 15 Conquest** So even with my conversion you will only allow an extra 3-4 conquest per week. This is not game breaking and could help us spend those soon to be useless emblems.