Huge fan of your guide! I studied it intensively on the wizards forum before it got closed. It's thanks to your guide my party was able to take down a young green dragon at lvl 3 without a single casualty. Can't wait to see what amazing combinations will show up next. Have you considered using true polymorphs secondary effect? That being that if the duration of an hour passes the transformation becomes permanent. Perhaps one could use this to shapeshift into an adult dragon for keeps and use the natural shapeshift ability of its new form/race to maintain its old class abilities and skills?
EDIT:
To elaborate a bit more on the idea:
A level 17 bard would get access to True polymorph as a level 9 spell
True polymorphs spell desc states and I quote: "If you concentrate on this spell for the full duration, the transformation becomes permanent."
The second part of this spell that's important is what you can transform into: "If you turn a creature into another kind of creature, the new form can be any kind you choose w hose challenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating). The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality. The target assumes the hit points of its new form"
Basically what this means is that level=CR, so a lvl 17 character can transform into a CR 17 creature like..oh, I don't know, an adult golden dragon would do.
So this means the spell not only provides us with a form of permanent transformation into another race/species, but also potentially a massive statbump on top of that. (Not to mention new abilities such as flight, breath attacks etc).
Now the downside of assuming a new form is that the spell notes that spells, actions and even talking can be limited or even impossible depending on what form you're in, but that's where the beauty comes in: not only can an adult dragon speak, use basic and advanced actions and even spells, they also have the ability to Change Shape naturally. Meaning a PC that became a dragon for keeps through this spell could simply Change Shape into the form they had before they were changed by the spell. (useful for entering dungeons and many other occurances).
An important detail regarding this is that it's mentioned that the statblock of the dragon sticks around when it shape changes, effectively providing a PC with 27 STR to use the adult golden dragon example.
The total result of this? A grappler with 27 STR, 25 CON, 14 DEX and 24 CHA (mentioning CHA for bards regarding spellcasting). Since AC is natural armor+Dex and an adult gold dragons AC is 19 we're dealing with a natural AC of 17 instead of 10, making it especially amazing for barbarians which suddenly end up with AC 26 (without a shield or any magic items!), a very potent AoE breath attack (hordes are grapplers weakspot afteral), the ability to Change Shape to avoid size restrictions and finally what makes it so amazing for us grapplers: The ability to grapple GARGANTUAN creatures (adult gold dragon is huge size after all), a massive STR bonus to grapple and all that on top of our athletic skill.
Ladies and gentlemen *puts down mug* We can grapple the Terrasque.
EDIT2: Note that this costs NO concentration after the spell becomes permanent and can be cast on allies as well!