Oh Hell Yeah!
Edit: Though I kinda wish they would include Clan stuff in MW 5: Mercenaries, like having to fight off the Clan Invasion (like in MW2: Mercenaries).
I can only assume but since it's called MW5 it will be based technologically off MW5 so all IS models could be re-used for MW5:Clans and the path may be open at some point through a DLC to combine the 2 games into one ( kind of what Total War Warhammer did that allowed to combine all 3 games into one massive map and giving you the option to use any army against any other army).
If they did that the sky is the limit and i'm getting all kinds of giddy if that actually happens and i get to pilot an IS Mech against the Clans My only gripe is that you apparently have to play the Smoke Jaguars for the campaign initially (booooh ) but i hope they will include a livery tool, so you can repaint the mechs in any Clan or even special Galaxies ( Ghost Bear Alpha Galaxy player here as well as a little Thirteenth Wolf Guards when i play Clans) you want for quick games or maybe some DLCs that will deal with other Clans.