Trump is not the Big Fish, here.
Nixon was in Watergate because Nixon, himself, was the maestro, conducting everything, telling everybody what to do.
What this investigation is doing is investigating the behind-the-scenes Machine that has been in operation for decades. The Trump kids etc. are just a part of the bigger investigation.
Yes, it is “possible” that the investigation will turn up OTHER stuff, some “smoking gun” that is accidentally discovered along the way. The grand jury’s job is to leave no stone uncovered or unturned. There are no rules of evidence laws re a grand jury. Some stuff may be inadmissible in trial, later, but that has nothing to do with this.
As was mentioned: if you want change, that can only happen in the voting booths.
And, yes, Nixon resigned before the Senate conducted a hearing to determine if Nixon had actually committed crimes that warranted Articles of Impeachment. CRIMES. AoI is a CRIMINAL indictment.