What option in Denver in case of delay
 

I am happy on one hand because I get a great award seat but I am also worried in case of delay. Both my departing and returning will have a layover in Denver for only about 1 hour 15 minutes. What happen if there is a delay and I cannot to the next flight in Denver? Due to the unfrequent flights available, do I have to connect to the same flight next day? can I connect to flight to nearby airport? Of course, I will prefer if I can connect to another airlines such as United, AA, etc without additional fee? any ideas?

75 minutes is plenty of time to connect in DEN. All Frontier flights are on the A concourse so you'll have no more than a 5-minute walk from gate to (furthest) gate.

In the highly unlikely event of a missed connection, Frontier will first try to put you on a later flight of theirs; if this is more than a couple of hours later, ask them to interline you on AA or US if they can. Frontier has an interline agreement with AA and US but not with UA, as I remember.

You have the best chance of interlining if you don't have checked baggage.