A modest proposal
Engaging in pure fantasy for the moment at the tail end of lunchtime, but if you will- given that there is a problem of too many unchallenged incumbents and uncompetitive districts, if I could wave my magic wand and change things, why not require that an incumbent's margin of victory improve by 1% over their previous year's result? (and perhaps 2% for Senators.) That is to say, if you won by 50.1% last year, you need 51.1% or better to stay in office (rather than simple plurality), and so on and so forth.
I see a few things from this:
1) Landslide initial victories would be pretty bad, as the bar would be set too high to repeat.
2) There would be an immediate appreciation for third parties, both to help reduce your initial winning tally and by challengers hoping to further sap the incumbent's chances of hitting their magic number.
3) Some hilarious "we have enough votes, stop voting for our candidate!" pleas near the end of uncompetitive races.
4) A lot more contested races and turnover.
Not sure if that would make district-line-writers more or less likely to try and pack as many partisans as possible into a district.
Ok, back to work.