Okay, so I know this could induce a 'you protest too much' response, but my statement is very much one of honesty - and it comes down to a single factor that there isn't much else there that I actually want.....that is available!
Starting off the collection with a Glacier Park Models, set a very high standard to follow and that has no-doubt limited some 'potential' purchases....infact it's left me wondering whether I should even part company with the Sunset MK-5:
It's a nice enough model, but lacks the detail of the GPM locos. New tender trucks, painting all the wheels/motion and a liberal weathering could improve things, but it's still would not be anywhere near the standard GPM set.
Two other loco's I've looked at include a PSC SP T-31 4-6-0 (Photo from Brass Trains):
One was on offer for about $850 this year at March-O-Meet, however model does come with some well known issues, the first being that when manufactured, DONGJIN based their drawing around the gauge being 4ft 8.5inches rather than 5ft (to match all other OW5) locos, so its firstly slightly too large, and secondly doesn't ride well on OW5 track. From memory there was only a few locos manufactured with this mistake, and they have subsequently been known as Q Scale locos. Thankfully the vendor alerted me to these issues and I declined the purchase. Nice model however and would have probably fitted in my fleet quite well.
Another potential which I probably should have snapped up a few years ago is the Glacier Park Northern Pacific S-4:
A vendor at 2024 and 2025 March O Meet had one for sale, and in retrospect, yes I should have purchased it - if it had been there this year, I probably would have, but alas it wasn't. It's not SP and I don't even know whether the NR would have had much of an interface with the SP, but it's a gorgeous model, as you would expect from Glacier Park. If one came up at a decent price, then yes I could be tempted.....but I'm not pursuing one.
Back to SP, a prototype I've grown interest in the Twelve-wheeler 4-8-0 set of locos they had, and Precision Scale have produced a model of the TW-8 class:
If a decent one came up then I'd debate it, but again its not one being actively pursued.
I do have a number of 'devils' sat on my shoulder who send me links to items for sale, including Sunset E8s.....and a pressure group even formed in Chicago this year to get me to purchase an SP 4-8-2 Mt-2 that was up for a good price.....but I resisted their best combined efforts!
The only other loco that is on the agenda is the SP P-10 Pacific:

PSC did produce a factory painted model and they do pop up on the second hand market from time to time, so I'm playing the waiting game. 3rd Rail are producing an all new model, but with it retailing at circa $2600, there needs to be a serious devaluation of the dollar for me to order one.
I guess the only other option to increase the fleet would be if any of the other releases of the GPM locos cam up for a decent price.....yeah I wouldn't mind another 2-8-0 with a different tender and potentially the snow plow fitted.....but chances of one coming up are very low.
So.....apart from the above there aren't really any other locos that have been manufactured that I really want, and unless Jimmy Booth decides to revitalise GPM models and start producing more RTR SP locos (that would be the dream), there won't be any new additions to the ever declining O scale market. Even diesel wise, there aren't any other prototypes that interest me, with the exception of the KM4000, which would really be out of time period modelled.
So....roll on E-bay listings materialising for a S-4, TW-8 and P-10 at ridiculously low prices, and me adding to this thread saying 'oh look what I've just purchased'.....
Cheers,
Stephen