Just a couple of things to ponder. Greek present is more likely to be indicating something durative than English, however, durative emphasis is fairly subtle and has to be judged by author and context.

Revelation's Greek is so bad that it's likely to be the product of a non-Greek speaker, very likely a Semitic mind, with some pretty rudimentary grasp of the language. Tradition holds that it was written by the Apostle John in exile under conditions where he didn't have a scribe, translator, or other help. While this could inspire one to speculate that this verb corresponds to a Hebrew imperfect or an Aramaic future, truly all this confirms for us is that the waters are muddy.

When things really are so muddy, the standard method is to directly render rubric by rubric, rather than introduce contextual nuance. So basically in Greek "A makes B" is rendered "A makes B" or "A is making B" or "A is going to make B"

So without knowledge of advance nuance due to the extreme rudimentary language, your options are:

"Behold, I make all things new" "Behold, I am making all things new"

Interestingly, "behold, I am going to make all things new" is not a likely translation. That use of a present construction to indicate future is English. When the Greek uses the present to indicate the future it normally refers to a generalized future - "help is coming" - that is vague about the details. (You don't know if the help has already started coming or if it will start coming soon, you only know it will get here.) So using the present to indicate explicitly that something hasn't started yet is rare. It's possible to describe such an action, but the author certainly doesn't care if you know the thing hasn't started yet.

So in English, if he said, "I make all things new" we would answer, 'duh, we know you do, you're God.' He's not expressing plan or intent, but rather stating present condition. In other words, one might be tempted to test if this is what Greek calls a gnomic present - not likely since he's just talked about the old passing away.

So the "I am making all things new" is the best translation, not because it expresses all the detailed meaning of the Greek, but rather because it contains the right amount of vagueness, LOL.

So there we have it. "I am making" is better than "I make" and "I am going to make" and now that you know this you can comfortably scratch your head in confusion and ask yourself what "I am making" means.