Apparently Delta doesn't know where its own focus cities are. Sorry, "flies to hubs and actual focus cities plus Cancun/Orlando/Las Vegas" does not make a market a focus city. If it did, MCO and LAS would be focus cities.
Now, they may *want* to make BNA/SJC/AUS focus cities, and RDU absolutely already is one.
But for SJC, unless United spilling the beans about their contract with Apple costs them their contract with Apple, saying SJC will get Apple traffic on DL is a stretch. That said, if DL *can* run SJV-PVG with a 339, they'll have a superior product vs. the current 788 setup from UA. Add in a Nerd Bird a few times per day to AUS, starting on an E75 if they have to (then upgauging to a 221/223 later) and you have a base to build on.
From the noise they're making about AUS, I'd say that (my) city falls squarely in "not a focus cities but will be within a year". The aforementioned SJC flights, plus 1-2x daily to DEN and BNA would probably be where they'd start (despite DEN already being a very competitive route). Maybe IND. All current hubs and (real) focus cities are already covered (and hubs are rapidly swapping 717s or E75s for 319s or bigger) and E75/717/A220 should be about the right size to start the point to point traffic off at. My bet is DL could command a slight price premium on DEN is they ran it with A220s, which would probably make the route work well enough to compete with other airlines' mainline (and United's regionals...it says something that UA can't keep mainline to their hub there).
Yes, DL is 4th place at AUS right now, but they were only 18k passengers behind United in April. That's 300 per day per direction, so adding the aforementioned routes would likely put them over the edge into 3rd place (in part because some of that traffic would be pulled from UA). Throw in an international flight on Delta metal and you're comfortably in third place. Hitting second place (currently held by American) would be a good bit more difficult, but their operation at RDU would put them in a comfortable second place here, so maybe they'll go for that.
As for BNA, DL is already the largest legacy (though not by much). Southwest has much more of a lock on the market there than here, but that's probably more of an opportunity for Delta. Will be interesting to see what routes they decide to start there. Because, sorry, MCO and CUN seasonally does not a focus city make.