Interesting. I was thinking of recommending Chorev. I don’t think its massiveness will be a problem, because each mitzvah is an individual self contained piece which has nothing to do with any of the others. Not only that, you don’t have to follow any order; you can pick whichever ones interest you the most. And I think 25 minutes is enough time to get through one of them – some of the shorter ones might even allow for multiple pieces per session. Also, if your considering something like Chorev, you can also consider something like Sefer Hachinuch or even the Rambam’s Sefer Hamitzvos, but I think Chorev would be the best of those for the intellectual stimulation you are looking for.
If you are considering Shemoneh Perakim, I would point out that it is not particularly long, so depending on your pace, you might well finish it in a couple of months, which if you are planning to have this chavrusa for the duration of the year would cause you to have to come up up with another idea in the middle of the year. Then again, if you do choose Shemoneh Perakim, and you are also doing Pirkei Avos with the Bartenura, if you finish Shemoneh Perakim worst comes to worst you can just segue into Pirkei Avos with the Rambam’s commentary. (Though I don’t know if you would want to be concurrently learning two different parts of Pirkei Avos, with different commentaries.)
(I was also going to suggest something in Tanach, but I assumed you wouldn’t want that. Bad assumption. I would have suggested something like Iyov which can be very intellectual especially in the discussion of why good things happen to bad people. But something like that probably wouldn’t work so well in a 25 minutes a week setup and it sounds like Navi was nixed anyway {although technically it’s not Navi}.)
Regarding mussar seforim, there is a broad spectrum. For example, something like Orchos Tzadikim, you would probably consider “fluffy”, Mesilas Yesharim (which you have already learned) and Ohr Yisrael probably less so, and something like Chovos Halevavos can be pretty intellectual, perhaps even bordering on philosophical.
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