then it would alternate between the two automatically until the next Axx command? How would you control the interval at which it alternated? For instance, how would you tell it to alternate on the next row, or after three rows, or something like that?

Doing it like in that comment would be the worst idea possible, since it will most definitely break a couple of hundred modules with multiple copypasted tempo commands on the same row. Nope nope nope, stateful commands like this are a very bad idea.
Either way, if anyone was to introduce such a command it will require a lot of designing: You cannot add it to any of the oldskool formats (MOD/XM/S3M/IT) since their command set is fixed. You could add it to the MPTM format, but then you have to keep in mind that the command, as suggested here, wouldn't work in modern tempo mode because the length of a row is defined by the tempo solely, and not by the tick count. And then there are the plugins which don't like tempo changes...

FWIW, to actually propose a sane and working way to implement this (which does <em>not</em> mean that I'll sit down and implement this right away, just before you ask) would be to extend the global and per-pattern time signature feature in modern tempo mode to have a shuffle property which would determine the percentage that each row contributes to the total beat length, e.g. if you have 4 rows per beat you could set the length for each row, where the default value would of course be 25%. This way, plugins that have trouble with tempo changes also wouldn't get confused because OpenMPT could already handle this internally.

What do you think about tempo maps implementation? Like this, for example:

TEMPO MAP #1
Row | Tempo
0 | 120
1 | 120
2 | 140
3 | 140
4 | 120
... | ...
---repeat---

And then use some command from pattern to activate or deactivate any tempo map.

As said, changing the tempo or ticks per row to achieve shuffling is semantically just wrong and will break many plugins. Furthermore, the way you propose is incompatible with e.g. tempo changes (T0x / T1x commands). Since it is rather unlikely that someone would want to change the shuffle amount in every other measure, I am pretty sure my suggestion is more practical and introduces less hassle.

Okay, I gave it a shot and implemented my idea. In the song properties, you can now click "Configure Swing" to set how much every row contributes to a beat. This can also be set for each pattern independently by using the time signature setup in the pattern properties. I'm not very happy yet about the swing config dialog, since setting the exact values you want using the sliders is probably not going to work very well, so please give me feedback how it works for and if you have any ideas how to improve the dialog.

New test build is available at http://buildbot.openmpt.org/builds/auto/openmpt-win32/ as always.

Interesting solution) And it looks like it works fine at this moment, but I think it will be hard to operate with lots of numbers of tempo parameters for beginner user.

That's why I'm looking for suggestions on how to improve it.

I’m not well aware of details because I’m the newcomer musician. Maybe we need to publish this solution on some forum, where lots of more professional people could offer their ideas?

I don't think anyone outside the OpenMPT world would care. Anyway, I'll close this issue here, if you have any ideas how to improve it further, open a new one.