Traditionally, the DA codex has been the rumour for everything coming to vanilla smurfs, lol.
it seems pretty solid for a while that it is going to be Tau then Eldar, as for after that......
Black Templars is by far the oldest and they're said again recently they don't want to roll it into the Space Marine codex because it would become bloated.
The question is whenever the new SM codex does show up what is it going to have to set it apart and make it competitive with the other marine codexs?
Cheaper HQs and cheaper troops? space marine codex is showing its age and these will certainly get cheaper IMO
access to upgrades such as FLAKK missiles and specialist equipment like DA? certainly
flyers? they already got the stormtalon which has since whored itself out to other codexs. i don't imagine SM will get another new flyer
new vehicles? they took a hit in 6th edition and if you really want to go mech BA is the way to go
dreads? this sounds more likely to me, either some kind of new dread or improved stats to the ironclad or something.
every new codex coming out has had some new great combos and competitive units. not to mention NEW units, i will be anxious to see what is going to set SM apart from their brothers and give me a reason to want to take them