With every other middlegame formation, I know more or less what I would do if I were given a load of free tempi, and thus, what I should aim for in the present. However, concerning a highly defensive Semi-Slav still with "fangs," it seems like there's not much I can do but twiddle my thumbs and hope Black opens up the position for me (which is of course a terrible strategy):
r1bq1rk1/ppn1bpp1/2p1pn1p/8/3P3B/2NBPN2/PPQ2PPP/R3K2R w KQ - 6 13
Black currently has five (!) defenders locking down d5, with the possibility of bringing in an additional rook or fianchettoed bishop if the current defenders were relocated/exchanged. Thus, I wouldn't have any idea how to launch a successful central takeover (in particular, whenever I push e3-e4 in these situations, my center pawns only seem to turn into a liability for me).
As well, the kingside is without any real defect to exploit, and starting a kingside pawn storm would (afaict) be to bet my whole position on an attack with less-than-ideal odds.
So, it seems to me that at present the queenside is White's best option to secure a weakness-free advantage. Let's say I get the opportunity for free to push my pawns and reposition my rook. The issue is that it seems Black often has the ability to liquidate the queenside at his will in his favor, for example:
[FEN "r1bq1rk1/ppn1bpp1/2p1pn1p/8/PP1P3B/2NBPN2/2Q2PPP/1R2K2R b KQ - 0 1"] 1... a5 2. bxa5 Ncd5 3. Nxd5 Nxd5 4. Bxe7 Qxa5+ 5. Nd2 Nxe7
Usually, however, Black commits to some expansion before White has the time to engineer a true queenside pawn storm, often in the form of a fianchetto for his weak bishop. This in turn tends to create a weakness on c6 that, personally, I'm glad to see, because it's at least something for White to begin to target.
But from my tabiya in the first diagram, how does White ideally want to take advantage of Black's position in the Semi-Slav, without giving himself weaknesses in the process, and without relying on Black's cooperation to loosen his own position?