One notorious issue for presentists (and other kinds of A-theorist) is the following: special relativity tells us (I gather) that among the slices of space-time that "look like time slices", there's no one that is uniquely privileged as "the present" (i.e. simulataneous with what's going on here-now). But the presentist says that only the present exists. So it looks like her metaphysics entails that there is a metaphysically privileged time-slice: the only one that exists. (Of course, I suppose the science is just telling us that there's no physically significance sense in which one is privileged, and it's not obvious the presentist is saying anything that conflicts with that. But it does seem worrying...)
One option is to retreat into "here-now"ism: the only things that exist are those that exist right here right now. No problems with relativity there.
I was idly wondering about the following line: say that it's (ontically) vague which time-slice is present, and so (for the presentist) say that it's ontically vague what exists. As I'm thinking of it, there'll be some kind of here-now-ish element to the metaphysics. From the point of view of a certain position p in space time, all that exists are those "time-like" slices of space time that contain the point, then it will be determinately the case that p exists. But for every other space-time point q, there will (I take it) be a reference frame according to which p and q are non-simultaneous. So it won't determinately be the case that q exists.
The details are going to get quite involved. I think some hard thinking about higher-order indeterminacy will be in order. But here's a quick sketch: choose a point r such that there's a choice of reference-frame that make q and r simultaneous. Then it sort of seems to me that, from p's perspective, the following should hold:
r doesn't exist
determinately, r doesn't exist
not determinately determinately r doesn't exist
The idea is that while r isn't "present" (and so fails to exist), relative to the perspective of some of the things that are present, it is present.
What I'd like to do is model this in a "supervaluation-style" framework like that one I talk about here. First, consider the set of all centred time-like-slices. It'll end up determinate that one and only one of these exists: but it'll be a vague matter which one. Let centred time-like-slice x access centred time-slice y iff the centre of y is somewhere in the time-slice x.
Now take a set of time-slices P which are all and only those with common centre p. These are the ontic candidates for being the present time. Next, consider the set P*, containing a set of time-slices which are all and only those accessed by some time-slice in P. And similarly construct P**, P*** etc etc etc.
Now, among space-time points, only the "here-now" point p determinately exists. All and only points which are within some some time-slice in P don't determinately fail to exist. All and only points which are within some time-slice in P* don't determinately determinately fail to exist. All and only points which are within some time-slice in P* don't determinately determinately determinately fail to exist. And so on. (If you like, existence shades of into greater and greater indeterminacy as we look further away from the privileged here-now point).
Well, I'm no longer sure that this deserves the name "presentism". Kit Fine distinguishes some versions of A-theory in a paper in "Modality and tense" which this view might fit better with (the Fine-esque way of setting this up would be to have the whole of space-time existing, but only some time-slices really or fundamentally existing. The above framework then models vagueness in what really or fundamentally exist). It is anyway up to it's neck in ontic vagueness, which you might already dislike. But I've no problem with ontic vagueness, and insofar as I can simulate being a presentist, I quite like this option.
There should be other variants too for different forms of A-theory. Consider, for example, the growing block view of reality (the time-slices in the model can be thought of as the front edges of a growing block: as we go through time, more slices get added to the model). The differences may be interesting: for the growing block, future space-time points determinately don't exist, but they don't det ...det fail to exist for some amount of iterations of "det"; while past space-time points determinately exist, but they don't det .... det exist for some amount of iterations of "det".
Any thoughts most welcome, and references to any related literature particularly invited!