Geography

[Exodus]

My other solution (which I now believe to be the best) was simply to make all cp tps invisible to the enemy team so that a gank via tp would not be so easy to spot without the minimap.

I highly doubt that this is going to fix this issue. First you have to consider, that the visible CP glow only has been added in v8.68. Do you feel, that ganking has become more difficult since then? This has been been added primarily to support more CP conquering. This works because the attacker now can see when defenders are incoming, which reduces the risk of attempting to conquer a CP.

I'd also argue, that just the simple act of teleporting to the CP to defend it, is not what's commonly called a gank. A gank normally has the element of suprise, but the CP glow won't change that. To see the glow, you have to see the CP, but when you see the CP, you'll also see the player that's teleporting there. It would only make a slight difference when just glancing in the direction of the CP, when a CP TP is currently in progress.
To wrap this up: I don't think this is going to solve the problem you were describing in your first post. I also don't want to lose the safer CP conquering, since I consider this a better direction for the overall game experience.