There are primarily two options:
An opportunity in the city (and surroundings) for progress in the spread of the gospel (see Acts 14:27; 1 Corinthians 16:9;2 Corinthians 2:12; Colossians 4:3);
A reassurance that the saints of Philadelphia will enter the New Jerusalem despite the trials that lie ahead (see Revelation 3:10).
Given the self-description Jesus gave of himself in Revelation 3:7, as well as the reference to little power
(Revelation 3:8) and upcoming trials
(Revelation 3:10), the second option is certainly the more acceptable. The first does not fit in the context of the letter.
8 “‘I know your works. Behold, I have set before you an open door, which no one is able to shut. I know that you have but little power, and yet you have kept my word and have not denied my name.