I wonder if CLE-IAD was just filler? Perhaps, Frontier figures CLE based pax might prefer driving one hour and get on Southwest's CAK-DCA (that launches in November) since DCA is more central to the Metro line and it's preferable to land in DCA than IAD. Despite building a focus in IAD, Even Frontier knows DCA is preferrable - otherwise it would cease DEN-DCA and move it to IAD.

US Air also flies the route (CAK-DCA) and is matching Southwest pricing, so together, that might be ample of low cost seats (in addition to Southwest's CLE-BWI), that Frontier figures it's not worthwhile to fly into IAD from CLE. Atleast with Frontier on CLE-ORD, it's not flying into an unfavorable airport of sorts, and possibly CLE demand to Chicago are greater than DC. Both seem driveable as well.

As far as CLE-Trenton, the frequency/timing before was awful but it was nice to have low fares between CLE and PHL/EWR region in a nonstop way. if Frontier decides to use the LGA slot pair for something else, CLE-TTN will be re-instated. Hopefully 5-6x weekly.

With Frontier offering a MEM-DFW-DEN, similarly, I think maybe it could do a TTN-CLE-IAH. N/S fares to IAH are expensive from PHL on US and United especially for same week trips without a weekend and one-ways, so if leisure pax is going to connect or stop anyways, Frontier could compete for that business and offer a 1-stop via CLE. CLE-Houston might be competitive with UA and if Southwest starts CLE-HOU, but I think Frontier might be equipped to compete against those two, as a secondary or third carrier. Houston seems to be a missing major market from CLE from Frontier, aside from the west coast markets which is much farther.