Ha... I have been down the last gun I will need to buy road a few times over! I will not bother to list all of the guns in my collection, but suffice it to say that "last gun" for me was only "last gun for a little while". I will go over my last 3 purchases which were after my initial "I think my collection is complete" stage.
The first slide off the slippery slope was my Springfield XDS in .45. My normal carry gun is a Kahr PM9, but I felt I wanted a little more power if I was only going to carry a single stack. The Kahr PM9 holds 6+1 and the Springfield XDS holds 5+1. I also liked the idea of having the grip safety as an extra measure against accidental discharges since I often pocket carry. I would classify this gun as a regret since the trigger is crap. I installed the Powder River Precision spring kit, but I have had a few FTF due to light strikes.
My second post "last gun" was the Sig P210 Legend in Silver. I kept looking at this gun over and over and really mulled it over for a bit. I have 2 other Sig P210s (a pre-owned "regular" P210 and the 50th Anniversary edition). I shoot the pre-owned one but could not bring myself to shoot the 50th anniversary edition, especially since it is brand new in box. The reason for getting the Legend is the beaver tail and the American mag release. The other reason is the P210 in silver is absolutely stunning!!!
My most recent post "last gun" was a Springfield Armory 1911 Loaded in 9mm. This was much more of an impulse buy on Gun Broker since it was listed $50 - $200 less other listings. Overall quality seems okay except for some sharp edges. I haven't had a chance to shoot it yet, but it seems like a well made gun.
Once you have most of your bases covered (as you have), you look for something special to be the icing on the cake or the cherry on top. I have to admit that the new Sig MPX is looking mighty attractive to me. I have no real need for it, but then again, how many guns does one really "need"?