I feel that it'd have more dramatic impact to have Kara visit the future rather than Mon-El just reappearing after a few episodes and explaining that he'd been gone for 1,000 years.
I'd like that a lot also -- poignant lovers separated in time would be much better -- and we did get the Legion foreshadowed already I believe. As for cost, just put the future in space most of the time -- no more expensive than the Daxams.
Oh and on Doomsday: Original Doomsday wasn't so much a character as a (rather stupid) plot device. To make sense, he needs enough of a travel power that you can't beat him by just getting him off the ground he needs to walk on -- as originally written, throw him into space and only the power of a bad writer's intervention could save him -- any GL takes him out in 30 seconds with no fuss.
Regarding Alex and Maggie... I've found that on all these shows, they're really much better if you fast forward through any scene where two characters are alone having a deeply emotional talk -- and you miss absolutely nothing. I don't feel deprived from not actually having watched any scene with Iris alone with Barry or her Dad on the Flash this season after the first couple episodes. Alex and Maggie seem to have nothing to do with the story aside from being Alex and Maggie. Couple, got it, don't really need anything else.