I've been looking forward to this one!
So let's see......
Season 19
Well this my fave Season ever, but there's still changes I would make. The obvious one would be to make Adric less annoying, Tegan less fiery and improve relations between all three companions. Even better, I'd drop the Adric character altogether, but then you wouldn't be killing him off at the end of Earthshock! I'd also improve the dialogue, especially in stories like Four To Doomsday as Tegan's in partcular was more cheesier than a deli counter at times!
I'd also make sure the stories were filmed in order so there isn't so many glaring differences in the hair styles and lengths. That way Davison wouldn't put in the 'bland' performance he described in Four To Doomsday. Though it wasn't that bad but you could see he was trying to find his feet in that story as it was first recorded.
Aside from that, the only big change I'd make in this Season would be to shift Earthshock to the end of the season which would create an even bigger impact. I often find that last episode worthy of an RTD Finale and Time Flight seems such an anti climax after it. And Time Flight itself I would replace all those dodgy mix of studio shots and Concorde taking off with CGI. Plus the studio airport backdrops used as they'd run out of money.
Season 20
For this Season I would have delayed Tegan's comeback until Snakedance so that Nyssa and The Doctor had some time together to work as a team. I think it worked in Arc Of Infinity. In some ways, I wouldn't have had her back at all, but then you wouldn't have had the Tegan/Turlough pairing which I think worked well.
Mawdryn Undead, i'd keep the Brig but have him in a different tole and bring back Ian Chesterton as originally intended.
Terminus I would beef up and inject some pace, it's so dull. Nyssa's exit would also be improved but the skirt drop scene I wouldn't, erm, drop, though a proper explanation of why she did it would be nice
The Guardians pigeon headwear would be replaced and Kamelion would be dumped after The King's Demons, so obvious it didn't work.
Season 21
Warriors Of The Deep. I'd drop the pantomime Myrka and replace it with something more convincing. Plus drop the kung foo kick by Doctor Solow. Plus reduce the lighting as the writer wanted.
The Awakening-Turlough, I'd give him more to do and stop locking him up! Plus keep Will Chandler as a companion, he was such a great character. Frontios I'd beef up as that slacks as well and make the Gravis less comical.
Resurrection Of The Daleks-Have more interaction between The Doctor and Lytton. It seemed in the following story between the two that they knew each other really well, but they hardly spoke in this one.
Planet Of The Daleks- Adopt Peri's Bikini as her regular costume from that point on......
Seriously, though, looking at the Season as whole, I'd have had Davison leave at the end of the Season. That way, the comings and goings wouldn't be so crammed into so few stories. With one extra story between Planet Of Fire and The Caves Of Androzani, my dream team of the Fifth Doctor and Peri would have an extra story. Turlough could have left one story later giving Planet Of Fire time to breath and not have so many requirements attached to it before it started. And yes, I'd have toned down The Master's panto version at this point.
Not only that, with Colin Baker having a dodgy debut, you could leave him until next season and you wouldn't have to wait months on end to see if he'd improve. It just seems weird having a New Doctor coming in at the end of another Doctor's Season. It was such a wrong decision to make, the first of many at this point. Plus I thought it was disrespectful to Davison to cut his last Season short in this way.
As for Davison's humour, would like to have seen more, but not too much to effect contrast with Baker and so his performance in Androzani stood out.