Any time that you modify the pattern of an antenna something suffers.
Now this isn't a change like going from an omni to a beam, but still a change.
Need to hit you with a couple questions first, to get some idea of what to possibly suggest.
Need ground level of town wanting to be covered, need ground level and tower height where antenna is to be installed.
That info will give me a better ideal of what will do the best overall.
As far as specifics of downtilt, and gain.
Imagine a coverage pattern of a unity gain omni directional antenna as a donut.
NOw as you add gain to the antenna, you are effectively crushing the donut.
The donut as you crush it will grow in diameter as it decreases in height, but the other thing that gets missed alot is the crush happens from the bottom up as well as the top down.
So the more gain, the flatter and higher the radiated pattern.
The reason that's important is that you can get to a point that you effectively talk over top a low area in the coverage pattern of the antenna.
So we bring in down tilt. This makes it like crushing the donut with two funnels with the walls at a specific angle (the down tilt angle).
You effectively tip the entire pattern down by X degrees in all directions. This can be used to drive signal down a high hill or mountain and into a valley.
But the coverage distance takes a hit in doing this.
But there is hope. Depending on a couple things.
The easiest way to 'have your cake and eat it too' is to run two antennas and a power divider or more correctly a directional coupler to send a specific amount of signal to each of the antennas.
THis is how you can fill a large multi-story building with coverage with one RF source and a BDA. The antenna system is nothing more than a bunch of ceiling mount antennas and properly selected RF directional couplers.
So the way that it would work is you have the top antenna as a standard pattern Omni, and then run a vertical with down tilt, or run a panel and put in mechanical down tilt when mounting it.
If you are wanting to cover one town in a valley this is actually a preferred method. You want to look at at the antenna beam width and the distance from the tower to the town and ensure that the beam width of the panel or even a corner reflector will cover the town or area in question. Now the selection of the signal tap on the directional coupler has to be carefully considered. It sounded like you are trying to get to the coast line with this, so I would with the current setup go take some measurements in dbm of the signal levels from the repeater to the areas of interest for coverage.
SO lets say that you have an average measurement of -80dbm in those areas. You can easily get away with a 10db tap, which would send 10 db of the total transmitter signal to the second antenna and allow for coverage in the valley below and the coast line as well.
There are some requirements to this sort of setup.
First is that the antenna system can't be shared, you will need to run it through a duplexer and not be on a multicoupler system. I don't believe that will be an issue here but you are significantly modifying the coverage footprint of anyone on that antenna system so others may or may not like the change.
Second is that you will be loosing something in the directional coupler. And that loss is bi-directional. so it can effect the receive as well as transmit. You can recover a lot of it with antenna gain, but there is only so much gain that's possible in an antenna. Transmit power can always be increased but that does nothing for the ability for the receiver to hear, and honestly, wide band, high gain amps in the receive line between the duplexer and receiver are courting a deaf receiver. Don't trry to do more than about 3 db of amp if at all.
You will need to consider every part of the antenna system and build for minimum loss. So you are going to need to run 7/8 or larger cable to get the most signal up and down the line. This of course depends on the height of the antenna on the top of the tower as you can place the directional coupler near the lower antenna and run a short jumper from teh tap port to the panel or corner reflector antenna.
Lastly, this ain't cheap. you can do a lot with a proper antenna system, but proper has dollar signs attached to it most of the time.
And of course, if you are trying to cover more than one town at the bottom of the mountain, it becomes increasingly difficult to do. You can come off the directional coupler into a 50/50 splitter and run two corner reflector antennas, but after that you get to a point of diminishing returns. But it would still be better than a flat onmi at the top and then a down tilt antenna some distance down the tower with a 360 coverage pattern.