I have seen the following sentences:
Let's see who wins.
Why not "who will win"?
Let it be decided tomorrow who will win.
Why not present here, then?
I think that both are possible.
I feel that there is a small nuance difference though:
Let's see who wins
The action is about to take place very soon.
Let it be decided tomorrow who will win
The action is delayed or postponed (by the speaker).
The present tense can be used to refer to future events, often when they form a schedule: ‘I fly to Paris on Tuesday, I’m in Madrid on Thursday and then on go on to New York for the weekend.’ The present tense in ‘Let's see who wins’ is not the same use, but it is probably more frequent than ‘Let's see who will win’, simply because it’s the kind of sentence that is likely to be found in an informal context. It’s simple, short and direct.
In my opinion:
You cannot see into the future, therefore sees
must represent the present.
To see who wins
describes the act of watching someone winning.
To see who will win
describes watching someone winning in the future (from the point of view of the watcher), so can be assumed to mean that the watcher is making a guess based on available evidence that a person will probably win, rather than watching someone who is actually winning.
In other words,
let's see who will win
could mean
let's make an educated guess as to who the winner will be
Whereas
let's see who wins
is more likely to mean what most of us think it does.
I think...
Regarding Let it be decided tomorrow who will win
, this actually only makes sense if someone is fixing the race, or deciding who will win before the race is run!
Of course we can take it less literally if it is spoken rather than written.
However, it would be better to say
Let the winner be decided tomorrow.