Not have them. Free up those zots for other things.
This is always an option. If they're just going to be like the horses in DAI I'd say go for it, they were a waste of time.
The trouble with improving the mounts is that you have to change a lot of the design of the game to MAKE it mount-friendly. This can be done, but if the ONLY reason why you're doing it is to make mounts worthwhile, you are (literally) putting the cart before the horse. So you kind of have to know "what are the design goals for the adventure areas in the next game?" before you even know whether mounts would make any dang sense in that context.
Now, if I had complete freedom and all the money in the world (but short of just saying "screw it, we're making an open game world") what I'd do is to make DA4 into a game where you're on the move a lot, as in, you're escorting a caravan or you're a small scouting force. The point is, you have to cover a lot of terrain over the course of the adventure. There would be two types of zones--adventure areas, which would be confined to an area where you'd made camp and set off to explore--and travel zones, which would be pretty to look at but fairly linear. All the stuff where you'd be constantly wanting to get on and off your horse would be confined to the adventure areas. You CAN explore them on horseback, but it's probably just more efficient to do it on foot. The travel zones are the opposite--you probably won't want to dismount at all.
So now the question is, why even have travel zones instead of just fast-travel points? Well, several reasons:
1. They're a perfect venue for some big, dramatic, open-area set piece encounters and plot developments.
2. They make the world feel really spacious.
3. You can have broader exploration-based gameplay here, instead of stuff that's based around poking your nose in every nook and cranny.
4. Having transitions like these makes it easier to control the pacing of the game (always good for story).
5. It adds more of a sense of "time passing" in the game than instantly teleporting between zones where it's always the same time of day.
You could also have a resource management aspect to it if you wanted to. Granted, I know that to some people this would be a bit Camping Simulator, but I'd enjoy it 
I'd make it so that you could zoom way, way out, too, and REALLY enjoy the scenery.
Transition zones could also be used to make game flow decisions. You only have to travel to an "undiscovered" area once, after that you can establish a fast-travel point and fast-travel there (not trying to be maliciously inconvenient). But you have some choice over which zones you go to, it's not completely linear. (And maybe you can find some secrets along the way.)
Basically, if you've ever played the now positively ancient original Pool of Radiance game, it'd be a bit like the "overland map" in that series of games. You could wander around semi-freely between zones, find interesting stuff, things would happen . . . it was pretty cool. And it made the game feel about 100 times as big as it would have if it had all been corridors.