Mmm. You need to be careful with conditions like this. Remember that people can always respond, and so whether Explorer is true on resolution may not be the same as whether Explorer was true on announcement. That's fine in cases like Crushing Avalanche or Needle Grass, but ones like this and Hidden Trails have the problem that the number or kind of targets is different in the two cases, but all targets have to be chosen when the spell is cast.

Suppose I cast this with Explorer false, targeting a creature of mine; but in response, someone else activates a Soratami Cloudskate so when it resolves, Explorer is true. What if the target doesn't have flying? For that matter, what if it does?

In this specific case, I think that's actually pretty funny. But in general it's a shame. Is there no trivial way to move the condition check to before the targeting?

This will also prove to be awkward if my opponent does not have any flying creatures in his deck and I know it. +1/+1 is a terrible trick for , but it's still something. When matched up against decks like that, players will feel encouraged to play less lands than all of their opponents, to either keep the trick active, or to keep the threat of the trick active. That seems contrary to the point of Explorer...

I think explorer should be assumed to determine spell mode on casting. I don't know how you template that (you could just add "as you cast this", but it would be a bit long), but since the condition is fairly simple, I think everyone would expect it to act like paying an additional free cost, not change while the spell is on the stack.

You could even make it optional if you're going to have radically different effects. (Or if it's a major theme of the set, have a special text box for it or something.)

Mmm... if the first ability is always on, and the second ability says "... destroy up to one target flying creature." then a lot of this these observations are made moot.

I believe the mode would be chosen upon casting of the spell. I mean, it's not something that's hard to tell whether you or an opponent has more lands in play. I will also admit to not being 100% familiar with the comprehensive rules, so I don't know whether it will actually check for conditions between casting and resolution unless something happens in between. I think it adds a little bit extra though if you could manage to turn Explorer off or on. Giving the casting player's creature flying and destroying it when he only meant to give it +1/+1, or to respond to it and getting +1/+1 to your own creature instead of seeing it destroyed.

I'm trying to figure out how to say "I don't think it matters much if it works within the realm of the rules or not. If enough people with a strong working knowledge of the Magic rules are confused as to how it works, then there's a problem." without coming off as snarky. Let's just assume I said that while smiling, but in a friendly way. Sound good?

After that, if you're cool with this thing changing modes mid-resolution, then I'm cool with this thing changing modes mid-resolution. I would suggest adding "...destroy target creature, if it is a flying creature..." to end the discussion about whether this card has modal problems, though.