Meh, neither Keldeo nor Landorus-I really need a ban, as they both have numerous checks and counters that stop them hard (cough Celebi cough), it's more of a predicting and scouting game with them. If Lando is Sheer Force (which it almost always is) Keldeo, Rotom-W, Chansey, Blissey, and other Special Walls deal perfectly with it, Keldeo is a bit more problematic, but it's usually Scarfed, and Tentacruel, Jellicent, Latias, and Politoed all wall it hard, let's hope for the best.
Just to expand on the pokemon you mentioned;
Assuming: Timid Life Orb Landorus-I w/ 252 Special Attack
Focus Blast v. 4 HP /252 Sp Def Bold Blissey: 36.34 - 43.09%
Focus Blast v. 4HP / 252 Sp Def Bold Chansey: 32.39 - 38.47%
Focus Blast v. 232 HP/ 0 Sp Def Modest Rotom-W: 63.87 - 75.58%
Earth Power v. 4 HP/ 0 Sp Def Timid Keldeo: 102.77 - 121.29%
So, if Landorus-I has Rock Polish, then Keldeo cannot deal with it at all, as it's outsped and OHKO'd 100% of the time. Blissey and Chansey can wall it OK, though it's worthy of note that Blissey has a 25% chance to be 2HKO'd with SR + 1 layer of Spikes - but otherwise, both of these are solid. My only qualms about Chansey/ Blissey is that they're not particularly good in the current meta so ehhh. And the standard bulky offense Rotom-W set is cleanly 2HKO'd, so it can only check Landorus-I; I suppose you could invest in special defense, but that means you're either not hitting as hard, or you're taking some important physical hits worse (like with Keldeo).
My main problem with Landorus-I is just the whole Rock Polish versus U-turn thing. If it hasn't got U-turn, then Celebi and Latias wall it easily - but then it can also sweep easier by outpacing everything after an RP. Then, if it has got U-turn, it's almost impossible to wall outside of Blissey/ Chansey (and even then it can rack up damage via U-turn + Hazards, to get them in to kill range). But, without RP, it's also easy to outspeed.
The problem with all this being, it's almost impossible to know what it's running, and yet with one moveslot change, Landorus-I plays (and is beaten) quite differently.
(also, CBtar with pursuit to trap Celebi/ Latias to set up a Landorus-I sweep can be pretty obnoxious).