TATL PDEW from MCI is over 370 so demand is obviously there. Perhaps FI just wasn't the best fit. Any chance MCI could interest BA, Condor, DL or even Aer Lingus (not sure if the A321NEO has the range)? If IND and CVG have service then why not KC? FI may have failed but i'm willing to bet MCI-LHR, FRA or AMS would do much better.
BA adding MCI-LHR nonstop service is a possibility as BA already serves LHR nonstop from some non-AA hub destinations in the US such as AUS, BWI, BOS, DEN, IAH, LAS, BNA, MSY, EWR, PIT, SAN, SFO, SJC, SEA, and IAD.
LH also already serves FRA nonstop from some non-UA hub airports in the US such as ATL, AUS, BOS, DFW, DTW, MIA, MCO, PHL, SAN, SEA, and TPA.
DE already serves ANC, BWI, FAI, LAS, MSP, MSY, PHX, PIT, PDX, and SEA nonstop from FRA on at least a seasonal basis.
KL already serves a few non-DL hub airports in the US nonstop from AMS such as ORD, IAH, LAS, SFO, and IAD. DL also already serve MCO and PDX nonstop from AMS, even though MCO and PDX aren't hubs for DL.
BA adding MCI-LHR nonstop service is more likely to happen than LH or DE adding MCI-FRA or KL adding MCI-AMS nonstop service since BA already has nonstop service to more US cities from LHR than KL does from AMS, DL does from AMS, LH does from FRA, or DE does from FRA. In addition to BA serving more U.S. cities nonstop from LHR, LHR is not as far east as AMS or FRA, and BA also already offers 1-stop connecting service to many European, African, and Middle Eastern destinations from the U.S. cities that it serves nonstop from LHR.