Tracking one-way fares likely won't be that effective as a MSP-BCN LIS-MSP open-jaw would qualify for cheaper roundtrip D1 fares and a one-way price tracker wouldn't pick up changes to roundtrip fares. Cheapest one-way D1 fares on routes are currently around $3800 while roundtrip D1 fares go for as low as $4500. It would be more effective to have roundtrip 1 week trips on both MSP-BCN and MSP-LIS tracked. Cheaper Z fares generally require mid-week travel.

Honestly, I wouldn't expect DL fares to ever go much cheaper from a hostage hub like MSP. Current cheapest Z fares out of Chicago (60 day AP, 7 day min stay ZLX67NDL fares below) go for roughly $3420 roundtrip. Expertflyer (which can do back-dated fare searches) shows some cheaper 90 day AP fares (ZNX96SDA fares) that were available for purchase in early February and were $240 less roundtrip. These fares were pulled on 2/11. They were not offered out of hostage hubs like MSP and DTW. I wouldn't expect DL fares to get any lower than $3200 price point from non hostage hub airports like ORD unless they see significant softening in demand.

In pre-Covid times, the legacy airlines would typically have deeply discounted business class fares in $2200 - $2400 roundtrip range for seasonal travel from later July through mid-August due to lack of business travel demand during this period (as it is a popular time for vacations). But times have changed post-Covid and the legacies are no longer offering such deeply discounted seasonal business class fares.