This is a cool idea, but it will be hard to make it work. For the various intricacies of casting spells always look here. So, let's see:
601.2. [...] Casting a spell follows the steps listed below, in order. [...]
601.2b If the spell is modal the player announces the mode choice (see rule 700.2). [...]
601.2c The player announces his or her choice of an appropriate player, object, or zone for each target the spell requires. [...]
601.3. Some spells specify that one of their controller's opponents does something the controller would normally do while it's being cast, such as choose a mode or choose targets. In these cases, the opponent does so when the spell's controller normally would do so.
601.3a If there is more than one opponent who could make such a choice, the spell's controller decides which of those opponents will make the choice.
So, the target opponent chooses one doesn't work, because the mode of the spell must be chosen before targets are determined. Luckily, 601.3a allows you to write "An opponent chooses one." 601.3 states that the opponent makes the decision at the time you would normally make it. "An opponent" doesn't specifiy which opponent, so 601.3a kicks in and states you can just pick one.
The next is Entwine and this is a more tricky one. The comp. rules text of Entwine is: “Entwine [cost]” means “You may choose to use all modes of this spell instead of just one. If you do, you pay an additional [cost].” What this actually means in the context of "An opponent chooses one" is not entirely clear. You get the option of choosing both modes when you would normally only be able to choose one, but you don't actually get to choose which mode is used, so does your opponent get to decide whether to Entwine the spell? I'd say that it's not possible to use Entwine in this way, unfortunately.
Thought Ransom - 1U
Instant (U)
Target spell's controller chooses one--
At Knifepoint - 2R
Sorcery (U)
Target opponent chooses one--
601.2. [...] Casting a spell follows the steps listed below, in order. [...]
601.2b If the spell is modal the player announces the mode choice (see rule 700.2). [...]
601.2c The player announces his or her choice of an appropriate player, object, or zone for each target the spell requires. [...]
601.3. Some spells specify that one of their controller's opponents does something the controller would normally do while it's being cast, such as choose a mode or choose targets. In these cases, the opponent does so when the spell's controller normally would do so.
601.3a If there is more than one opponent who could make such a choice, the spell's controller decides which of those opponents will make the choice.
So, the target opponent chooses one doesn't work, because the mode of the spell must be chosen before targets are determined. Luckily, 601.3a allows you to write "An opponent chooses one." 601.3 states that the opponent makes the decision at the time you would normally make it. "An opponent" doesn't specifiy which opponent, so 601.3a kicks in and states you can just pick one.
The next is Entwine and this is a more tricky one. The comp. rules text of Entwine is: “Entwine [cost]” means “You may choose to use all modes of this spell instead of just one. If you do, you pay an additional [cost].” What this actually means in the context of "An opponent chooses one" is not entirely clear. You get the option of choosing both modes when you would normally only be able to choose one, but you don't actually get to choose which mode is used, so does your opponent get to decide whether to Entwine the spell? I'd say that it's not possible to use Entwine in this way, unfortunately.
Completed sets:
Iamur — The Underwater Set
Overworld — Pirates vs. Octopuses
Esparand — The Sands of Time
Unfinished Sets:
Siege of Ravnica — Eldrazi in Ravnica
Shandalar — The Mana Set
Iamur Reimagined — Iamur v2
You can find more creative projects on my page Antaresdesigns!