All of SIU 8 being done in the next couple of decades is unrealistic, but I think the Millington to Covington section has a good chance of being out to contract as soon as TDOT is done with SIU 7 if they can ever get the FEIS done.
My guess is that TDOT will want to designate I-269 over the whole route, rather than holding off north of I-40; US 51 is a legal interim terminus for an Interstate designation, and having two disconnected routes with the same number creates headaches for incident management and route inventory purposes, especially when they're in the same county.
I can see the argument for holding off the redesignation for now and keeping it as stealthy as possible, since I-40 west traffic might think I-269 would get them to I-69 south of Memphis today and end up very confused in the middle of nowhere in Marshall County, nowhere near any casinos or anything else to speak of. But when 2018 rolls around the odds of similar motorist confusion are low since there's no logical reason for long-distance traffic to take I-40 west to (nonexistent) I-69 north (anyone headed to Dyersburg would have cut over on US 412), so waiting longer doesn't make a lot of sense.