Upcoming gym change/ rebalance

I don't know but it's pretty wise to save a significant portion of dust for awhile?

I would also spend on a few only Pokemon: Blissey, Dragonite (very moveset and IV dependent esp the rarity of Dratini candy in your area) , Tyranitar, Snorlax (w/ HB or BS since both movesets are versatile enough as attacking and defensive moves) Gyarados (DT w/O or HP or B/HP only), Lapras, and Vaporeon (w/ the exception of WP). (You can debate Rhydon, but from what I see only one is needed w/ MS/SE or E w/ great IVs & maybe Machamp should be added w/ C/CC or DP if you need attackers into Blissey?) Since you have coins and stardust to collect till that rework comes. I hope they don't render most our Pokemon we powered up useless or most of us are waving goodbye.

Moves with STAB is always a safe bet

I would speculate that the gym rework will be far more than just a change in movesets or adjustments of move specifications.
We have had movesets change, and moves have had stat adjustments in the past - but these were not considered to be a 'gym rework'.

I think that the whole gym system will get a significant rework, and anything we know about gyms will become obsolete.
If this is the case, every single aspect of the gym system will need to be evaluated from scratch...as if it is a whole different game that uses our current collection of mons.

I suspect that after the gym rework, battles will be not even be comparable to what we have now.

Of course, this is going to cause quite a stir, and there will be a backlash from some parts of the entire PoGo community - but I think we can all agree that something significant needs to change...I think the gym rework will bring that.

Will GP and all other fan sites need to do a lot of work to give the players information about a whole new system?
Yes absolutely.

This is all just speculation though.

Even small changes could have a massive effect on moveset rankings so I imagine it will/would change things significantly.

Similarly to sOmeasnguy I have elected to pretty much only spend on good base stat mons since I don't think base stats will change and such mons as Dragonite should weather whatever storm may come. I'm avoiding powering CP based defender talents like Gyrados.

My guess is that STAB will be devalued to some extent to improve the comparative usefulness of non stab movesets. This could effect some stab based niche mons. But honestly it's very hard to speak on since we have no idea at all what the changes may be.

I'm also trying (unsuccessfully) to avoid powering Blissey since she is an outlier and outliers have a lessened chance to survive "balancing" intact.

My hope is that the balancing is a defender formula that takes mon level, defensive stats, CP, and maybe other factors into account for placement and first mon dropped from defeated gyms.

One simple gym rebalance that Niantic can implement is to create a variety of "defensive profiles" that is randomly applied to each defender at the start of gym battles. This way, no two battles are exactly the same.

For example, they can create a defensive profile where defenders can launch their quick moves more often, instead of the norm but still abide by the move speed and cool down rules. They can create another where defenders don't always launch charge moves when it becomes ready. They can create another where defenders dodge occasionally, etc., etc.

This way certain defensive profiles applied to certain mons will give it advantages while other defensive profiles will weaken certain mons. I think this will really change it up without much work.

I don't expect the Gym Fundamentals to Change Honesty.

I expect the Rework to be something out of Ingriss where you get notifcations your gym is under attack and go defend it.

PMG isn't turn based like Pokemon. I don't think turned base fits the mobile world as much. And there are limits on buttons/use on a Phone so to me a more complicated attack sequence is out.

I don't see a better way for stacking pokemon in gyms either. CP is the best of bad ways. The only way they could maybe make it better is to create a new stat called DP and then add a flat bonus +400 or penalty -200 to the CP.

But even then, that wont change much, since Vaperon will still be a strong defender and the reality is in general in pokemon some pokemon > others.