Even the last ones, when they brought them back for three years, had respectable attendances.
Celtic identity wouldn't be forced for people from Celtic countries. "I'm a Celt" is not the first thing you'd say to somebody. Naturally, we'd say "I'm Welsh/Irish/Scottish etc." but to refer to ourselves as Celts is pretty common. Actually, when meeting another Celt from one of the other countries it wouldn't be at all unusual to say "oh a fellow Celt, I'm xyz". Just as an example, I watch a lot of union from France and keep an eye out for and watch a lot of Vannes matches. Why? Because Vannes are Breton and therefore Celtic. So there's a connection. At their matches you see Celtic flags (the flags of the six Celtic nations in one flag). It wouldn't be a forced thing at all to push something because it was Celtic. There would be logistical issues. For example, home grounds would have to be alternated, play one match in Cardiff, next in Edinburgh, next in Dublin etc. otherwise people would perceive it as Welsh/Scottish/Irish if they only played in one location. Obviously we're not going to get tied to such a team in the way we would to our national teams but the point is it wouldn't immediately feel unnatural. You're from about as far away as it's possible to go in England from a Celtic country so it's perfectly reasonable you're not aware of this.
A good example of why GB doesn't cut it for us is look at the rugby union Lions. They're pretty much exclusively referred to as the Lions, the badge is a shield with the logos of the four countries involved on it and the colour, although it's red is pretty neutral because it's a very different shade of red to the Wales shirt. And everyone, English, Scottish, Irish, Welsh who is a union fan supports them (including me). The rugby league Lions are almost exclusively referred to as GB (so immediately Ireland is excluded), and GB is basically a synonym for England (I know technically it means the main island in this collection of isles but it in common parlance doesn't mean that), the colours are basically England, the badge is very English. There's absolutely nothing wrong with that. That's not my point. But it either is England with the odd Celt bolted on, which is its current format, or its something more in-line with the union Lions which it isn't at the moment. Also, if Scotland which is appearing more and more likely goes independent, and the desire for independence is growing in Wales too (this is getting dangerously close to far too political a discussion for a sports forum - and I hold my hand up for being responsible for that but there's no way I can explain what I'm getting at in any other way), where does that leave a GB team? The union Lions continue. The league Lions, what happens? The league Lions can carve out their own identity and don't need to be remotely modelled on the union Lions. In fact, they shouldn't be other than looking at the outline of a blueprint. Another good example is the England cricket team. Personally I would rather watch paint dry than cricket but I do know a lot of people in Wales find the English cricket team obnoxious because technically it is England and Wales but no reference is ever made to that second part. I know plenty of people back home who are ambivalent about England winning at soccer, union or league but who really can't stand to see the cricketers winning for the reason I've outlined.
That said, it depends on what the desired outcome is. If league is happy enough to continue as pretty much an M62 sport in the NH, and in some ways I could understand the rationale for wanting this because various attempts have been made to get the game going in Wales, Scotland and Ireland, fair enough. Continue with GB as it is. It doesn't inhibit expansion into London and other areas of England. If it wants to make a serious drive for getting traction in Wales, Scotland and Ireland then GB almost certainly needs a makeover. Certainly if it wants to include Ireland it does. Personally I don't think it would take a huge effort. Call it the "Lions", soften the badge and colours and it's done. But I can completely understand people not wanting to do this too.
In terms of Merseyside/Greater Manchester, I'm in a similar boat to you with the Celtic name. I'm from relatively near both but I'm not from either and given Merseyside and Greater Manchester were formed and Cheshire expanded in the decade I was born, Saints, Wigan, Warrington etc. have never been in Lancashire within my living memory. To me Saints are from Merseyside (I know that's heresy here but I'm a foreigner so what do I know
), Wigan from Greater Manchester and Warrington from Cheshire. That said, I am aware that within league circles all three are very much still part of Lancashire and there is a rivalry still with Yorkshire but I don't know whether that's to do with the age of the league fans I know or not.
As per the political side of GB, I hope people reading this will accept that I'm using it here to highlight a sporting point rather than trying to trigger people. I have my political viewpoint but I respect others. Out of courtesy, apart from in relation to GB in this context, I won't be drawn on that viewpoint though as politics has a habit of very quickly derailing good conversation.
In terms of the Celtic thing, it would be interesting to get the views of the other Celts on this forum. I know there's a Scottish guy who is on the podcast (sorry mate, I feel like I should know your name but I don't) and Tallaght Tiger is Irish. Any way of tagging them into the discussion?