We're upgrading to an ETC board with no existing ETC Net infrastructure in the theater. I want to do it right.

The Ion has a program on it called GCE (Gateway configuration Editor) which allows you to edit the function of the node.  I agree that purchasing a 4 port node would help out with adding the third universe to your system.  However I wouldn't route the Ion sACN traffic through the wireless router, more so I would recommend using a standard ethernet switch, and only using the wireless router (connecting the console to the WAN port) for the aRFR/iRFR connection.  With the Ion on the LAN side of the router, the constant sACN traffic will be transmitted by the router, slowing down aRFR/iRFR connections...  Also of note, the ION by default is set up to be the DHCP server (assigns network IP addresses), if you had the ion on the LAN side of the router, you would have to turn of either the DCHP server on the router, or the console.

once the node is in place, you can run cat5e lines to where ever you need to run the console from and not have to reroute your DMX traffic.  Besides the nodes, no other specific hardware is needed for the design you have drawn up.

Long story short I think you're on the right path.  Keep asking the questions!

If you purchased a 4 port gateway you would have up to 6 universes as there are 2 built into ion.  When we bought our Eos, we purchased a 4 port gateway.  After just a short time we wished we hadn't. 

I would seriously consider where you need DMX in your facility. Don't purchase a 4 port gateway just because it sounds good.   I feel that 4 single port gateways, in a permanent install, is better than one 4 port.

For ease of operation we now are running 16 Universes in our main theater.  We never come close to filling them but is actually more cost effective for us to use a single port gateway than an opto-splitter.  We did, however, put a lot of thought into our infrastructure and have POE switches throughout the facility.

Like Glenn mentioned a network switch would be needed but it is pretty simple to configure and ETC Tech Services would be able to help you with any installation hurdles.  There are other considerations with Single port gateways, like POE. 

I would contact your local ETC representative for information on what would be best in your facility as every venue is different,

You can configure any port or all ports as Input or Output depending on your needs.

If you purchased a 4 port gateway you would have up to 6 universes as there are 2 built into ion.  When we bought our Eos, we purchased a 4 port gateway.  After just a short time we wished we hadn't. 

I would seriously consider where you need DMX in your facility. Don't purchase a 4 port gateway just because it sounds good.   I feel that 4 single port gateways, in a permanent install, is better than one 4 port.

For ease of operation we now are running 16 Universes in our main theater.  We never come close to filling them but is actually more cost effective for us to use a single port gateway than an opto-splitter.  We did, however, put a lot of thought into our infrastructure and have POE switches throughout the facility.

Like Glenn mentioned a network switch would be needed but it is pretty simple to configure and ETC Tech Services would be able to help you with any installation hurdles.  There are other considerations with Single port gateways, like POE. 

I would contact your local ETC representative for information on what would be best in your facility as every venue is different,

You can configure any port or all ports as Input or Output depending on your needs.

What was your reason for regretting a 4-port gateway?  Was it just that it was all locked into one place?

We need three universes in the booth to support what we already have installed, and then there's talk of buying a handful of LED fixtures (with fine arts money, whereas the new board comes from building money) which could end up anywhere.  I just don't want to be boned when they buy 10 LED fixtures and we can only use them from the cove or 1LX because we don't have any other places to plug in.  The way things are set up now, we aren't able to easily run DMX to the floor.

I realize that 6 universes is total overkill, but I really like the idea of a single ethernet cable that needs to run home instead of two lines of DMX as well as an ethernet cable, for when we set up our tech tables or otherwise move the board to a non-standard position.  And when it comes to cost, it looks like a single four-port gateway is more cost efficient than two doubles or four singles.

But if we're not ready/able to run dedicated ethernet ports, we would then be forced to run a DMX line from the booth to our backstage area.  And a 100' ethernet cable is much cheaper than a 100' DMX cable.  But still, we could get additional gateways (I'm really not sure what dollar amount will cause the higher ups to start questioning every choice) with the 4-port gateway setup and still do that.  I'm mainly just thinking out loud here, preparing to send my boss a revised proposal.  I need to make sure we think far enough into the future to not waste this opportunity we have now.

While the node isn't 100% necessary, (an opto splitter and adjusting the addresses on either the movers or the scrollers would also do the trick).  If you're going to invest 10k in a console, thinking about the gateway and additional network ports wouldn't be a bad way to make incremental investments in your system.  One of the other benefits to the cat5 distribution and Entour (portable) gateways, is DMX when you need it where you need it.   I turned on a new HS performing arts center last fall, with all the money spent on installed single port gateways (1 per elec, each corner of the deck, box booms, and catwalks), much operating capital could have been saved by still pulling the cat5 (the cheap part) putting a jack on the end of said cable.  

With the adoption of RDM and the fact that sACN isn't just an ETC protocol (grandMA now has it on their desks for example), I would be looking into considering how and where I could use these in future expansion.