What others said about research and effort needed to model and program a plane. It is a lot of work. So if you prefer those planes just state I wish we had X because I like it. Most people are fine with that.
As for saying it will fill out a planeset, well needs a little more thought. Remember there are two types of game plane, main arena and special events. We have massive gaps in some of our plane sets (Japan and Russia really need more planes to round them out) and some very specific gaps in others. In regards to the MA, most prefer LW and not sure there are all that many planes that could be added that would be used there or contribute.
B-29 is out for various reasons. A-26 Invader would be a good add for MA.
For special events there are quite a few planes that are needed to fill out early and mid war planesets.
For the Germans the He 111 would be nice to have for Battle of France, Battle of Britain, Invasion of Russia, etc. It would probably be my number one pick for the Germans for special events use.
For the Russians see other threads discussing what the Russian set really needs. It definitely needs early war fighters (1941-1942) and ingeniousness bombers (Pe-2 comes to mind).
For the Japanese, again see other threads. At the very least they need an early to midwar bomber. Ki-67 is not very useful for special events set in early and midwar. Also would be nice to have the Ki-43 but not really necessary since A6Ms can sub for it. Overall Japanese definitely can use some more planes.
The British, more bombers would help in many situations. Right now you just have the Boston IIIs and the Lancaster. I can think of a slew of bombers to debate about for Special events (Wellington, Manchester, Hudson, Blenheim, Whitley, etc., etc. )
As for the Americans it would be nice to have a 1941-1943 torpedo plane.