I’d go custom myself but some key points, and I’ll be in the minority here and say avoid Webers (for gas, they aren’t really that great compared to most).
Cast iron grates are a must. And pretty tight spacing so shrimp cannot fall through. Asparagus will but that’s about the “hole size” you want. (to cook asparagus of course lay it perpendicular to the openings).
Surface area. HUGE. HUGE HUGE. One of my biggest pet peeves about Weber’s, not enough surface area. My Kenmore has enough surface area to put 2 whole turkeys on the grill. (oh yeah, you need a tall lid).
A top rack that can be sturdy wire. A place to keep things warm, to slow cook baked potatoes, etc. Make it LARGE. Large enough that it basically, when the lid is closed, extends to match the surface area of the lower grill. You can never have enough space.
I like a metal pan that supports volcanic briquettes. They get changed out once a year. Juices drip on them, you get the smoky and this is a huge difference worth exploring. IT costs nothing but another 4 notches, one at each corner, slightly below the burner level but such that the briquettes fit in the holes of the burner.
Have the heat adjustable left to right, not front to back. You can then put veggies on the left side, and meat on the right and have the right temp for both. The front back is just nonsense as who wants to reach OVER one set of flames to get to something else. Cook “side to side” and not “Front to back”.
Think about cleaning. How will you disassemble it?? You need to get the cast iron grates off, you need to get the burners out, you need to get the catch tray out and you need to get the catch cup out. Plus you’ll want to hose out the lid so need some drainage for gallons of water. Where will the venturi’s be (hopefully with the burners as one piece) as you’ll need to hose them out because spiders etc will build small structures in there and mess up your airflow over time.
Handles. WOOD or hard plastic. Metal handles on a grill lid?? How stupid is that? You don’t want to be forced to wear gloves all the time to open/close just to check on things.
You want NON-metallic metal pieces. If a magnet sticks to it, avoid it. Aluminum, most stainless steels. Heavy duty, when you touch something you want it moving exactly how you want, no “wiggle room”.
For all pieces (screws, hinges, burners), that will be directly exposed to heat or that will get filled with meat juices, fats, etc and thus wear out or get destroyed during cleaning/disassembly, buy 2X and keep in a drawer. And make sure you are buying standardized everything every chance you get. No "oh that was last year’s model, this year we have a 4-screw hinge which is “much better”. Yeah except that I’m trying to replace it on a pre-existing built in grill that only has 3 holes in very very specific places. Morons!
Storage. First is “accessible storage” and I’m talking easy to reach hooks for your grill scraper, for your tongs, for the spatula. Second is “non-accessible” which tends to be doors underneath. I AVOID any “non-accessible” myself. Everything lives inside and comes out when it is time to play.
My $0.04