How about this one, then?
 

c) 2/10 from A's 20. At the last second before the ball is snapped A88 realizes he should be on the field and steps on the field just inside the sideline and catches a quick pass from the QB and runs down field and scores a TD.


Do we have a foul? If so what is the foul?
Where will the yardage be marched off from and how many yards?
What will be the next down and distance and from where?

Illegal participation. Foul is enforced from the previous spot. 2 and 20 at the A10.

Illegal formation, 7-2-1.

It would be IP if you decide it violates 9-6-4d:

To use a player, replaced player, substitute, coach, trainer or other attendant in a substitution or pretended substitution to deceive opponents at or immediately before the snap or free kick.

The more interesting question here is not the type of foul but when the ball is declared dead. We had at least one thread on here where one or more posters' answer was that the play is whistled dead as soon as you realize what's gone on (i.e. after the ball becomes live but before the down's normal termination), but that the foul is one deemed to occur as the ball was put in play, and the ball is retroactively ruled not to have been put in play.

So, just to liven things up, what if there's no such whistle and instead of a touchdown, A88 goes downfield a little and is interfered with by B21 attempting to catch the pass? (And the cool thing is, we still haven't specified whose code the game's played under, and it's an interesting question regardless. For Canadian football, assume the wing officials had "put up the gates".)

That is not the proper procedure for this foul. Illegal participation is a live ball foul and it does not kill the play.

Welpe, WHY is it IP? I agree it is IP

3.5.7 says

Furthermore, the act
of his coming onto the field must not deceive the defensive team.


He came onto the field late and was subbing for someone that left and this deceived the defense, the act did, doesn't have to be intent!