Right off the bat, you're going to want a BIG panel. You already need 30 spaces just for the circuits you have right now, so a 30 will result in being full on day one. I want to see you have more of a "reserve" than a measly 12, but prices get exponential beyond 42 spaces, so I would stop there and use a subpanel later if you need still more.
Double-stuff breakers (like you have now) won't work anymore. Modern codes require AFCI and/or GFCI on virtually all circuits, and those are not available in double-stuff (as you can see from the one GFCI or AFCI in your panel). So if you get a "20-space/40-circuit" panel, you will have painted yourself into a corner (again). Only spaces will suffice. Also, this panel is a fluke, in that it makes efficient use of space with 2-pole double-stuffs. That's not possible anymore, not even on a GE panel.
If a big panel is too tall, consider regrading the earth around the panel, so you're standing higher on the wall.
Your new panel absolutely needs a main breaker; it's no longer allowed to hot-feed six large breakers as is found in this old "Rule of Six" panel. The size is decided by the power company, not you. Do not let this deter you from getting a large panel. This can be an annoyance if the Power Company wants a smaller breaker (say 125A) than anyone sells in a 42-space panel; however main breakers can be changed, and many "main lug" panels have space to upgrade a main breaker.
Now, I don't know what's on the other side of that wall. But I would put the panel indoors if I could help it. Why? Keeps it out of the weather. Weatherproof panels aren't, although yours has done far better than I usually see.
The cable run between meter and main breaker will be totally unfused. If it has a meltdown or short, unlimited amps will flow, unlimited heat will be made. It can make sense to fit an external main breaker even if you mount the panel indoors. At the least, make sure you are using all metal conduit - PVC fed by an electrical arc will just burn.
I would buy accessory ground bars for the new panel (or buy a panel that comes with), and separate neutrals and grounds (even though this is not strictly required in a new panel). I am a huge fan of wiring the neutral-ground equipotential bond - first a screw thread is wholly insufficient for 500A flow, it will just vaporize; and second, I like being able to clamp an ammeter around the N-G bond.
Looking at this panel I'm concerned with the lack of visible grounds. Now, the metal conduit is a viable ground path, so maybe the ground wires are attached to the junction box on the other side of the wall, that'd be fine. But I also see an awful lot of neutral wires as compared with the hots. I wonder if all the Romex cables are terminated in the box on the other side of the wall, and each wire was extended with black and white (THWN I hope). And then they used white for ground, "since they go to the same bar" being their logic. That is improper, they should've used green (or stripped the wires bare).