They're going to be in a bit of a bind with the cap. They might let Sustr go, and likely Kunitz as well, but they'll need to give Miller a $2 mil or so raise. WPG 1st would be included for sure, but I'm not sure what else would be required. Ideally the Blues would like to unload a bit of salary in return, so you're looking at Sobotka as the obvious choice - TB would still be saving over $1.5 mil in cap space with Sobotka signed for just 2 more years, and he could easily be their 3rd line center with Miller as their 2nd line center. As for the plus required on top of that, perhaps the Blues do have to add a prospect like Thompson in order to unload Sobotka as well (Sobotka alone still has value, but with a team like TB who is up against the cap, they would likely want a better prospect in return for taking a player with a bit of salary).
Johnson is a solid 2nd line center. His 70 point season appears to be an outlier, as otherwise he's about a 50 point player (over the past 5 seasons, Johnson paced for 50, 77, 45, 56, and 50 points per season). That's about the same amount of production that Stastny produced for us (51, 63, 50, 53 points per season). Given that Johnson is RH, that adds a bit more value. At $5 mil per season over the next 6 seasons (through age 33), I don't think you're looking at a contract that you're regretting, as long as Johnson keeps his historical ppg pace. He would lock down the 2nd center position, with Thomas having the potential to take over that position within the next couple of years, sliding Johnson down to the 3rd line.
If I were DA, I would certainly start with WPG 1st + Sobotka, and I'm sure TB would counter with an addition of one of Kostin/Thompson (Kyrou and Thomas are a no go). Perhaps DA could get TB down to a lesser prospect, but who knows. I think this is a move DA will certainly consider, though, over the summer.
By substituting Sobotka with Johnson, that increases the cap hit by just $1.5 mil, still giving the Blues a shot at going after JT.
Could you imagine Tavares/Schenn/Johnson down the middle over the next 6 years (assuming Schenn is signed)? Or Tavares/Johnson/Thomas, with Schenn moving to RW at some point? That would be incredible.
Of course, Tavares is a pipedream, but if that happens, I'd send Thompson + WPG 1st + Sobotka for Johnson all day long - that opens our window immediately and keeps it open for the next 5+ years.
I'll stop daydreaming now...